Go Working Group R. Gieben Internet-Draft August 25, 2018 Intended status: Informational Expires: February 26, 2019 Learning Go draft-learning-go-00 Preface The source of this book [1] is written in mmark [2] and is converted from the original LaTeX source [3]. _All example code used in this book is hereby licensed under the Apache License version 2.0._ This work is licensed under the Attribution-NonCommercial- ShareAlike 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/ [4] or send a letter to Creative Commons, 171 Second Street, Suite 300, San Francisco, California, 94105, USA. The following people made large or small contributions to earlier versions of this book: Adam J. Gray, Alexander Katasonov, Alexey Chernenkov, Alex Sychev, Andrea Spadaccini, Andrey Mirtchovski, Anthony Magro, Babu Sreekanth, Ben Bullock, Bob Cunningham, Brian Fallik, Cecil New, Cobold, Damian Gryski, Daniele Pala, Dan Kortschak, David Otton, Fabian Becker, Filip Zaludek, Hadi Amiri, Haiping Fan, Iaroslav Tymchenko, Jaap Akkerhuis, JC van Winkel, Jeroen Bulten, Jinpu Hu, John Shahid, Jonathan Kans, Joshua Stein, Makoto Inoue, Marco Ynema, Mayuresh Kathe, Mem, Michael Stapelberg, Nicolas Kaiser, Olexandr Shalakhin, Paulo Pinto, Peter Kleiweg, Philipp Schmidt, Robert Johnson, Russel Winder, Simoc, Sonia Keys, Stefan Schroeder, Thomas Kapplet, T.J. Yang, Uriel"\dagger", Vrai Stacey, Xing Xing. "Learning Go" has been translated into (note that this used the original LaTeX source). o Chinese, by Xing Xing, 这里是中文译本: http://www.mikespook.com/learning-go/ [5] I hope this book is useful. Miek Gieben, London, 2015. Gieben Expires February 26, 2019 [Page 1] Internet-Draft Learning Go August 2018 This book still sees development, small incremental improvements trickle in from Github. Miek Gieben, London, 2017. Learning Go's source has been rewritten in mmark2 [6], but did not see any other changes. This books translates cleanly into an RFC- like document [7]. Miek Gieben, London, 2018. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on February 26, 2019. Copyright Notice Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Gieben Expires February 26, 2019 [Page 2] Internet-Draft Learning Go August 2018 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6 1.1. How to Read this Book . . . . . . . . . . . . . . . . . . 7 1.2. Official Documentation . . . . . . . . . . . . . . . . . 8 2. Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2.1. Hello World . . . . . . . . . . . . . . . . . . . . . . . 9 2.2. Compiling and Running Code . . . . . . . . . . . . . . . 10 2.3. Variables, Types and Keywords . . . . . . . . . . . . . . 10 2.3.1. Boolean Types . . . . . . . . . . . . . . . . . . . . 11 2.3.2. Numerical Types . . . . . . . . . . . . . . . . . . . 11 2.3.3. Constants . . . . . . . . . . . . . . . . . . . . . . 12 2.3.4. Strings . . . . . . . . . . . . . . . . . . . . . . . 12 2.3.5. Runes . . . . . . . . . . . . . . . . . . . . . . . . 13 2.3.6. Complex Numbers . . . . . . . . . . . . . . . . . . . 13 2.3.7. Errors . . . . . . . . . . . . . . . . . . . . . . . 13 2.4. Operators and Built-in Functions . . . . . . . . . . . . 13 2.5. Go Keywords . . . . . . . . . . . . . . . . . . . . . . . 14 2.6. Control Structures . . . . . . . . . . . . . . . . . . . 15 2.6.1. If-Else . . . . . . . . . . . . . . . . . . . . . . . 15 2.6.2. Goto . . . . . . . . . . . . . . . . . . . . . . . . 16 2.6.3. For . . . . . . . . . . . . . . . . . . . . . . . . . 16 2.6.4. Break and Continue . . . . . . . . . . . . . . . . . 16 2.6.5. Range . . . . . . . . . . . . . . . . . . . . . . . . 17 2.6.6. Switch . . . . . . . . . . . . . . . . . . . . . . . 18 2.7. Built-in Functions . . . . . . . . . . . . . . . . . . . 19 2.8. Arrays, Slices, and Maps . . . . . . . . . . . . . . . . 20 2.8.1. Arrays . . . . . . . . . . . . . . . . . . . . . . . 20 2.8.2. Slices . . . . . . . . . . . . . . . . . . . . . . . 21 2.8.3. Maps . . . . . . . . . . . . . . . . . . . . . . . . 23 2.9. Exercises . . . . . . . . . . . . . . . . . . . . . . . . 25 2.9.1. For-loop . . . . . . . . . . . . . . . . . . . . . . 25 2.9.2. Answer . . . . . . . . . . . . . . . . . . . . . . . 25 2.9.3. Average . . . . . . . . . . . . . . . . . . . . . . . 26 2.9.4. Answer . . . . . . . . . . . . . . . . . . . . . . . 26 2.9.5. FizzBuzz . . . . . . . . . . . . . . . . . . . . . . 27 2.9.6. Answer . . . . . . . . . . . . . . . . . . . . . . . 27 3. Functions . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.1. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 30 3.2. Functions as values . . . . . . . . . . . . . . . . . . . 31 3.3. Callbacks . . . . . . . . . . . . . . . . . . . . . . . . 31 3.4. Deferred Code . . . . . . . . . . . . . . . . . . . . . . 32 3.5. Variadic Parameter . . . . . . . . . . . . . . . . . . . 34 3.6. Panic and recovering . . . . . . . . . . . . . . . . . . 34 3.7. Exercises . . . . . . . . . . . . . . . . . . . . . . . . 36 3.7.1. Average . . . . . . . . . . . . . . . . . . . . . . . 36 3.7.2. Answer . . . . . . . . . . . . . . . . . . . . . . . 36 3.7.3. Bubble sort . . . . . . . . . . . . . . . . . . . . . 37 Gieben Expires February 26, 2019 [Page 3] Internet-Draft Learning Go August 2018 3.7.4. Answer . . . . . . . . . . . . . . . . . . . . . . . 37 3.7.5. For-loop II . . . . . . . . . . . . . . . . . . . . . 38 3.7.6. Answer . . . . . . . . . . . . . . . . . . . . . . . 38 3.7.7. Fibonacci . . . . . . . . . . . . . . . . . . . . . . 38 3.7.8. Answer . . . . . . . . . . . . . . . . . . . . . . . 38 3.7.9. Var args . . . . . . . . . . . . . . . . . . . . . . 39 3.7.10. Answer . . . . . . . . . . . . . . . . . . . . . . . 39 3.7.11. Functions that return functions . . . . . . . . . . . 39 3.7.12. Answer . . . . . . . . . . . . . . . . . . . . . . . 39 3.7.13. Maximum . . . . . . . . . . . . . . . . . . . . . . . 40 3.7.14. Answer . . . . . . . . . . . . . . . . . . . . . . . 40 3.7.15. Map function . . . . . . . . . . . . . . . . . . . . 40 3.7.16. Answer . . . . . . . . . . . . . . . . . . . . . . . 41 3.7.17. Stack . . . . . . . . . . . . . . . . . . . . . . . . 41 3.7.18. Answer . . . . . . . . . . . . . . . . . . . . . . . 41 4. Packages . . . . . . . . . . . . . . . . . . . . . . . . . . 43 4.1. Identifiers . . . . . . . . . . . . . . . . . . . . . . . 45 4.2. Documenting packages . . . . . . . . . . . . . . . . . . 46 4.3. Testing packages . . . . . . . . . . . . . . . . . . . . 47 4.4. Useful packages . . . . . . . . . . . . . . . . . . . . . 49 4.5. Exercises . . . . . . . . . . . . . . . . . . . . . . . . 51 4.5.1. Stack as package . . . . . . . . . . . . . . . . . . 51 4.5.2. Answer . . . . . . . . . . . . . . . . . . . . . . . 51 4.5.3. Calculator . . . . . . . . . . . . . . . . . . . . . 52 4.5.4. Answer . . . . . . . . . . . . . . . . . . . . . . . 52 5. Beyond the basics . . . . . . . . . . . . . . . . . . . . . . 54 5.1. Allocation . . . . . . . . . . . . . . . . . . . . . . . 55 5.1.1. Allocation with new . . . . . . . . . . . . . . . . . 55 5.1.2. Allocation with make . . . . . . . . . . . . . . . . 55 5.1.3. Constructors and composite literals . . . . . . . . . 56 5.2. Defining your own types . . . . . . . . . . . . . . . . . 57 5.2.1. More on structure fields . . . . . . . . . . . . . . 58 5.2.2. Methods . . . . . . . . . . . . . . . . . . . . . . . 59 5.3. Conversions . . . . . . . . . . . . . . . . . . . . . . . 60 5.3.1. User defined types and conversions . . . . . . . . . 61 5.4. Exercises . . . . . . . . . . . . . . . . . . . . . . . . 62 5.4.1. Map function with interfaces . . . . . . . . . . . . 62 5.4.2. Answer . . . . . . . . . . . . . . . . . . . . . . . 62 5.4.3. Pointers . . . . . . . . . . . . . . . . . . . . . . 63 5.4.4. Answer . . . . . . . . . . . . . . . . . . . . . . . 63 5.4.5. Linked List . . . . . . . . . . . . . . . . . . . . . 63 5.4.6. Answer . . . . . . . . . . . . . . . . . . . . . . . 64 5.4.7. Cat . . . . . . . . . . . . . . . . . . . . . . . . . 66 5.4.8. Answer . . . . . . . . . . . . . . . . . . . . . . . 66 5.4.9. Method calls . . . . . . . . . . . . . . . . . . . . 70 5.4.10. Answer . . . . . . . . . . . . . . . . . . . . . . . 70 6. Interfaces . . . . . . . . . . . . . . . . . . . . . . . . . 71 6.1. Which is what? . . . . . . . . . . . . . . . . . . . . . 72 Gieben Expires February 26, 2019 [Page 4] Internet-Draft Learning Go August 2018 6.2. Empty interface . . . . . . . . . . . . . . . . . . . . . 73 6.3. Methods . . . . . . . . . . . . . . . . . . . . . . . . . 74 6.4. Methods on interface types . . . . . . . . . . . . . . . 74 6.5. Interface names . . . . . . . . . . . . . . . . . . . . . 75 6.6. A sorting example . . . . . . . . . . . . . . . . . . . . 75 6.7. Listing interfaces in interfaces . . . . . . . . . . . . 77 6.8. Introspection and reflection . . . . . . . . . . . . . . 77 6.9. Exercises . . . . . . . . . . . . . . . . . . . . . . . . 80 6.9.1. Answer . . . . . . . . . . . . . . . . . . . . . . . 80 6.9.2. Pointers and reflection . . . . . . . . . . . . . . . 81 6.9.3. Answer . . . . . . . . . . . . . . . . . . . . . . . 82 7. Concurrency . . . . . . . . . . . . . . . . . . . . . . . . . 82 7.1. Make it run in parallel . . . . . . . . . . . . . . . . . 85 7.2. More on channels . . . . . . . . . . . . . . . . . . . . 85 7.3. Exercises . . . . . . . . . . . . . . . . . . . . . . . . 86 7.3.1. Channels . . . . . . . . . . . . . . . . . . . . . . 86 7.3.2. Answer . . . . . . . . . . . . . . . . . . . . . . . 86 7.3.3. Fibonacci II . . . . . . . . . . . . . . . . . . . . 88 7.3.4. Answer . . . . . . . . . . . . . . . . . . . . . . . 89 8. Communication . . . . . . . . . . . . . . . . . . . . . . . . 90 8.1. io.Reader . . . . . . . . . . . . . . . . . . . . . . . . 91 8.2. Some examples . . . . . . . . . . . . . . . . . . . . . . 92 8.3. Command line arguments . . . . . . . . . . . . . . . . . 92 8.4. Executing commands . . . . . . . . . . . . . . . . . . . 93 8.5. Networking . . . . . . . . . . . . . . . . . . . . . . . 93 8.6. Exercises . . . . . . . . . . . . . . . . . . . . . . . . 94 8.6.1. Finger daemon . . . . . . . . . . . . . . . . . . . . 94 8.6.2. Answer . . . . . . . . . . . . . . . . . . . . . . . 95 8.6.3. Echo server . . . . . . . . . . . . . . . . . . . . . 96 8.6.4. Answer . . . . . . . . . . . . . . . . . . . . . . . 96 8.6.5. Word and Letter Count . . . . . . . . . . . . . . . . 98 8.6.6. Answer . . . . . . . . . . . . . . . . . . . . . . . 98 8.6.7. Uniq . . . . . . . . . . . . . . . . . . . . . . . . 99 8.6.8. Answer . . . . . . . . . . . . . . . . . . . . . . . 99 8.6.9. Quine . . . . . . . . . . . . . . . . . . . . . . . . 99 8.6.10. Answer . . . . . . . . . . . . . . . . . . . . . . . 100 8.6.11. Processes . . . . . . . . . . . . . . . . . . . . . . 100 8.6.12. Answer . . . . . . . . . . . . . . . . . . . . . . . 101 8.6.13. Number cruncher . . . . . . . . . . . . . . . . . . . 102 8.6.14. Answer . . . . . . . . . . . . . . . . . . . . . . . 103 9. References . . . . . . . . . . . . . . . . . . . . . . . . . 106 9.1. Informative References . . . . . . . . . . . . . . . . . 106 9.2. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 107 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 111 Gieben Expires February 26, 2019 [Page 5] Internet-Draft Learning Go August 2018 1. Introduction Is Go an object-oriented language? Yes and no. ------------------------------------------------------------ Frequently asked questions, Go Authors The Go programming language is an open source project language to make programmers more productive. According to the website [go_web] "Go is expressive, concise, clean, and efficient". And indeed it is. My initial interest was piqued when I read early announcements about this new language that had built-in concurreny and a C-like syntax (Erlang also has built-in concurrency, but I could never get used to its syntax). Go is a compiled statically typed language that feels like a dynamically typed, interpreted language. My go to (scripting!) language Perl has taken a back seat now that Go is around. The unique Go language is defined by these principles: Clean and Simple Go strives to keep things small and beautiful. You should be able to do a lot in only a few lines of code. Concurrent Go makes it easy to "fire off" functions to be run as _very_ lightweight threads. These threads are called goroutines in Go. Channels Communication with these goroutines is done, either via shared state or via channels [csp]. Fast Compilation is fast and execution is fast. The aim is to be as fast as C. Compilation time is measured in seconds. Safe Explicit casting and strict rules when converting one type to another. Go has garbage collection. No more "free()" in Go: the language takes care of this. Standard format A Go program can be formatted in (almost) any way the programmers want, but an official format exists. The rule is very simple: The output of the filter "gofmt" _is the officially endorsed format_. Gieben Expires February 26, 2019 [Page 6] Internet-Draft Learning Go August 2018 Postfix types Types are given _after_ the variable name, thus "var a int", instead of "int a". UTF-8 UTF-8 is everywhere, in strings _and_ in the program code. Finally you can use "\Phi = \Phi + 1" in your source code. Open Source The Go license is completely open source. Fun Programming with Go should be fun! As I mentioned Erlang also shares some features of Go. A notable difference between Erlang and Go is that Erlang borders on being a functional language, while Go is imperative. And Erlang runs in a virtual machine, while Go is compiled. 1.1. How to Read this Book I've written this book for people who already know some programming languages and how to program. In order to use this book, you (of course) need Go installed on your system, but you can easily try examples online in the Go playground. All exercises in this book work with Go 1, the first stable release of Go -- if not, it's a bug. The best way to learn Go is to create your own programs. Each chapter therefore includes exercises (and answers to exercises) to acquaint you with the language. Each exercise is either _easy_, _intermediate_, or _difficult_. The answers are included after the exercises on a new page. Some exercises don't have an answer; these are marked with an asterisk. Here's what you can expect from each chapter: basics We'll look at the basic types, variables, and control structures available in the language. functions Here we look at functions, the basic building blocks of Go programs. packages We'll see that functions and data can be grouped together in packages. We'll also see how to document and test our packages. Gieben Expires February 26, 2019 [Page 7] Internet-Draft Learning Go August 2018 beyond-the-basics We'll create our own types. We'll also look at memory allocations in Go. interfaces We'll learn how to use interfaces. Interfaces are the central concept in Go, as Go does not support object orientation in the traditional sense. concurrency We'll learn the "go" keyword, which can be used to start function in separate routines (called goroutines). Communication with those goroutines is done via channels. communication Finally we'll see how to interface with the rest of the world from within a Go program. We'll see how to create files and read and write to and from them. We'll also briefly look into networking. 1.2. Official Documentation There is a substantial amount of documentation written about Go. The Go Tutorial [go_tutorial], the Go Tour (with lots of exercises) and the Effective Go [effective_go] are helpful resources. The website http://golang.org/doc/ [8] is a very good starting point for reading up on Go. Reading these documents is certainly not required, but it is recommended. When searching on the internet use the term "golang" instead of plain "go". Go comes with its own documentation in the form of a program called "godoc". If you are interested in the documentation for the built- ins, simply do this: % godoc builtin To get the documentation of the "hash" package, just: % godoc hash To read the documentation of "fnv" contained in "hash", you'll need to issue "godoc hash/fnv" as "fnv" is a subdirectory of "hash". Gieben Expires February 26, 2019 [Page 8] Internet-Draft Learning Go August 2018 PACKAGE DOCUMENTATION package fnv import "hash/fnv" Package fnv implements FNV-1 and FNV-1a, non-cryptographic hash ... 2. Basics I am interested in this and hope to do something. ------------------------------------------------------------ On adding complex numbers to Go, Ken Thompson In this chapter we will look at the basic building blocks of the Go programming language. 2.1. Hello World In the Go tutorial, you get started with Go in the typical manner: printing "Hello World" (Ken Thompson and Dennis Ritchie started this when they presented the C language in the 1970s). That's a great way to start, so here it is, "Hello World" in Go. package main <1> import "fmt" <2> // Implements formatted I/O. /* Print something */ <3> func main() { <4> fmt.Printf("Hello, world.") <5> } Lets look at the program line by line. This first line is just required _1_. All Go files start with "package ", and "package main" is required for a standalone executable. "import "fmt"" says we need "fmt" in addition to "main" _2_. A package other than "main" is commonly called a library, a familiar concept in many programming languages (see Section 4). The line ends with a comment that begins with "//". Next we another comment, but this one is enclosed in "/*" "*/" _3_. When your Go program is executed, the first function called will be "main.main()", which mimics the behavior from C. Here we declare that function _4_. Gieben Expires February 26, 2019 [Page 9] Internet-Draft Learning Go August 2018 Finally we call a function from the package "fmt" to print a string to the screen. The string is enclosed with """ and may contain non- ASCII characters _5_. 2.2. Compiling and Running Code To build a Go program, use the "go" tool. To build "helloworld" we just enter: % go build helloworld.go This results in an executable called "helloworld". % ./helloworld Hello, world. You can combine the above and just call "go run helloworld.go". 2.3. Variables, Types and Keywords In the next few sections we will look at the variables, basic types, keywords, and control structures of our new language. Go is different from (most) other languages in that the type of a variable is specified _after_ the variable name. So not: "int a", but "a int". When you declare a variable it is assigned the "natural" null value for the type. This means that after "var a int", "a" has a value of 0. With "var s string", "s" is assigned the zero string, which is """". Declaring and assigning in Go is a two step process, but they may be combined. Compare the following pieces of code which have the same effect. var a int a := 15 var b bool b := false a = 15 b = false On the left we use the "var" keyword to declare a variable and _then_ assign a value to it. The code on the right uses ":=" to do this in one step (this form may only be used _inside_ functions). In that case the variable type is _deduced_ from the value. A value of 15 indicates an "int". A value of "false" tells Go that the type should be "bool". Multiple "var" declarations may also be grouped; "const" (see Section 2.3.3) and "import" also allow this. Note the use of parentheses instead of braces: Gieben Expires February 26, 2019 [Page 10] Internet-Draft Learning Go August 2018 var ( x int b bool ) Multiple variables of the same type can also be declared on a single line: "var x, y int" makes "x" and "y" both "int" variables. You can also make use of _parallel assignment_ "a, b := 20, 16". This makes "a" and "b" both integer variables and assigns 20 to "a" and 16 to "b". A special name for a variable is "_". Any value assigned to it is discarded (it's similar to "/dev/null" on Unix). In this example we only assign the integer value of 35 to "b" and discard the value 34: "_, b := 34, 35". Declared but otherwise _unused_ variables are a compiler error in Go. 2.3.1. Boolean Types A boolean type represents the set of boolean truth values denoted by the predeclared constants _true_ and _false_. The boolean type is "bool". 2.3.2. Numerical Types Go has most of the well-known types such as "int". The "int" type has the appropriate length for your machine, meaning that on a 32-bit machine it is 32 bits and on a 64-bit machine it is 64 bits. Note: an "int" is either 32 or 64 bits, no other values are defined. Same goes for "uint", the unsigned int. If you want to be explicit about the length, you can have that too, with "int32", or "uint32". The full list for (signed and unsigned) integers is "int8", "int16", "int32", "int64" and "byte", "uint8", "uint16", "uint32", "uint64", with "byte" being an alias for "uint8". For floating point values there is "float32" and "float64" (there is no "float" type). A 64 bit integer or floating point value is _always_ 64 bit, also on 32 bit architectures. Note that these types are all distinct and assigning variables which mix these types is a compiler error, like in the following code: Gieben Expires February 26, 2019 [Page 11] Internet-Draft Learning Go August 2018 package main func main() { var a int var b int32 b = a + a b = b + 5 } We declare two different integers, a and b where a is an "int" and b is an "int32". We want to set b to the sum of a and a. This fails and gives the error: "cannot use a + a (type int) as type int32 in assignment". Adding the constant 5 to b _does_ succeed, because constants are not typed. 2.3.3. Constants Constants in Go are just that --- constant. They are created at compile time, and can only be numbers, strings, or booleans; "const x = 42" makes "x" a constant. You can use _iota_ to enumerate values. const ( a = iota b ) The first use of "iota" will yield 0, so "a" is equal to 0. Whenever "iota" is used again on a new line its value is incremented with 1, so "b" has a value of 1. Or, as shown here, you can even let Go repeat the use of "iota". You may also explicitly type a constant: "const b string = "0"". Now "b" is a "string" type constant. 2.3.4. Strings Another important built-in type is "string". Assigning a string is as simple as: s := "Hello World!" Strings in Go are a sequence of UTF-8 characters enclosed in double quotes ("). If you use the single quote (') you mean one character (encoded in UTF-8) --- which is _not_ a "string" in Go. Once assigned to a variable, the string cannot be changed: strings in Go are immutable. If you are coming from C, note that the following is not legal in Go: Gieben Expires February 26, 2019 [Page 12] Internet-Draft Learning Go August 2018 var s string = "hello" s[0] = 'c' To do this in Go you will need the following: s := "hello" c := []rune(s) <1> c[0] = 'c' <2> s2 := string(c) <3> fmt.Printf("%s\n", s2) <4> Here we convert "s" to an array of runes _1_. We change the first element of this array _2_. Then we create a _new_ string "s2" with the alteration _3_. Finally, we print the string with "fmt.Printf" _4_. 2.3.5. Runes "Rune" is an alias for "int32". It is an UTF-8 encoded code point. When is this type useful? One example is when you're iterating over characters in a string. You could loop over each byte (which is only equivalent to a character when strings are encoded in 8-bit ASCII, which they are _not_ in Go!). But to get the actual characters you should use the "rune" type. 2.3.6. Complex Numbers Go has native support for complex numbers. To use them you need a variable of type "complex128" (64 bit real and imaginary parts) or "complex64" (32 bit real and imaginary parts). Complex numbers are written as "re + im""i", where "re" is the real part, "im" is the imaginary part and "i" is the literal '"i"' ("\sqrt{-1}"). 2.3.7. Errors Any non-trivial program will have the need for error reporting sooner or later. Because of this Go has a builtin type specially for errors, called "error". "var e error" creates a variable "e" of type "error" with the value "nil". This error type is an interface -- we'll look more at interfaces in Section 6. For now you can just assume that "error" is a type just like all other types. 2.4. Operators and Built-in Functions Go supports the normal set of numerical operators. See Table 1 for lists the current ones and their relative precedence. They all associate from left to right. Gieben Expires February 26, 2019 [Page 13] Internet-Draft Learning Go August 2018 +------------+--------------------------+ | Precedence | Operator(s) | +------------+--------------------------+ | Highest | "* / % << >> & &^" | | | `+ - | | | "== != < <= > >=" | | | "<-" | | | "&&" | | Lowest | || | +------------+--------------------------+ Table 1: Operator precedence. "+ - * /" and "%" all do what you would expect, "& | ^" and "&^" are bit operators for bitwise _and_ bitwise _or_ bitwise _xor_ and bit clear respectively. The "&&" and "||" operators are logical _and_ and logical _or_ Not listed in the table is the logical not "!" Although Go does not support operator overloading (or method overloading for that matter), some of the built-in operators _are_ overloaded. For instance, "+" can be used for integers, floats, complex numbers and strings (adding strings is concatenating them). 2.5. Go Keywords Let's start looking at keywords, Table 2 lists all the keywords in Go. +-------------+----------------+-----------+-------------+----------+ | "break" | "default" | "func" | "interface" | "select" | | "case" | "defer" | "go" | "map" | "struct" | | "chan" | "else" | "goto" | "package" | "switch" | | "const" | "fallthrough" | "if" | "range" | "type" | | "continue" | "for" | "import" | "return" | "var" | +-------------+----------------+-----------+-------------+----------+ Table 2: Keywords in Go. We've seen some of these already. We used "var" and "const" in the Section 2.3 section, and we briefly looked at "package" and "import" in our "Hello World" program at the start of the chapter. Others need more attention and have their own chapter or section: o "func" is used to declare functions and methods. o "return" is used to return from functions. We'll look at both "func" and "return" in detail in Section 3. Gieben Expires February 26, 2019 [Page 14] Internet-Draft Learning Go August 2018 o "go" is used for concurrency. We'll look at this in Section 7.3.1. o "select" used to choose from different types of communication, We'll work with "select" in Section 7.3.1. o "interface" is covered in Section 6. o "struct" is used for abstract data types. We'll work with "struct" in Section 5. o "type" is also covered in Section 5. 2.6. Control Structures There are only a few control structures in Go. To write loops we use the "for" keyword, and there is a "switch" and of course an "if". When working with channels "select" will be used (see Section 7.3.1). Parentheses are not required around the condition, and the body must _always_ be brace-delimited. 2.6.1. If-Else In Go an "if" looks like this: if x > 0 { return y } else { return x } Since "if" and "switch" accept an initialization statement, it's common to see one used to set up a (local) variable. if err := SomeFunction(); err == nil { // do something } else { return err } It is idomatic in Go to omit the "else" when the "if" statement's body has a "break", "continue", "return" or, "goto", so the above code would be better written as: if err := SomeFunction(); err != nil { return err } // do something Gieben Expires February 26, 2019 [Page 15] Internet-Draft Learning Go August 2018 The opening brace on the first line must be positioned on the same line as the "if" statement. There is no arguing about this, because this is what "gofmt" outputs. 2.6.2. Goto Go has a "goto" statement - use it wisely. With "goto" you jump to a label which must be defined within the current function. For instance, a loop in disguise: func myfunc() { i := 0 Here: fmt.Println(i) i++ goto Here } The string "Here:" indicates a label. A label does not need to start with a capital letter and is case sensitive. 2.6.3. For The Go "for" loop has three forms, only one of which has semicolons: o "for init; condition; post { }" - a loop using the syntax borrowed from C; o "for condition { }" - a while loop, and; o "for { }" - an endless loop. Short declarations make it easy to declare the index variable right in the loop. sum := 0 for i := 0; i < 10; i++ { sum = sum + i } Note that the variable "i" ceases to exist after the loop. 2.6.4. Break and Continue With "break" you can quit loops early. By itself, "break" breaks the current loop. Gieben Expires February 26, 2019 [Page 16] Internet-Draft Learning Go August 2018 for i := 0; i < 10; i++ { if i > 5 { break <1> } fmt.Println(i) <2> } Here we "break" the current loop _1_, and don't continue with the "fmt.Println(i)" statement _2_. So we only print 0 to 5. With loops within loop you can specify a label after "break" to identify _which_ loop to stop: J: for j := 0; j < 5; j++ { <1> for i := 0; i < 10; i++ { if i > 5 { break J <2> } fmt.Println(i) } } Here we define a label "J" _1_, preceding the "for"-loop there. When we use "break J" _2_, we don't break the inner loop but the "J" loop. With "continue" you begin the next iteration of the loop, skipping any remaining code. In the same way as "break", "continue" also accepts a label. 2.6.5. Range The keyword "range" can be used for loops. It can loop over slices, arrays, strings, maps and channels (see Section 7.3.1). "range" is an iterator that, when called, returns the next key-value pair from the "thing" it loops over. Depending on what that is, "range" returns different things. When looping over a slice or array, "range" returns the index in the slice as the key and value belonging to that index. Consider this code: list := []string{"a", "b", "c", "d", "e", "f"} for k, v := range list { // do something with k and v } First we create a slice of strings. Then we use "range" to loop over them. With each iteration, "range" will return the index as an "int" Gieben Expires February 26, 2019 [Page 17] Internet-Draft Learning Go August 2018 and the key as a "string". It will start with 0 and "a", so "k" will be 0 through 5, and v will be "a" through "f". You can also use "range" on strings directly. Then it will break out the individual Unicode characters ^[In the UTF-8 world characters are sometimes called _runes_ Mostly, when people talk about characters, they mean 8 bit characters. As UTF-8 characters may be up to 32 bits the word rune is used. In this case the type of "char" is "rune". and their start position, by parsing the UTF-8. The loop: for pos, char := range "Gő!" { fmt.Printf("character '%c' starts at byte position %d\n", char, pos) } prints character 'G' starts at byte position 0 character 'ő' starts at byte position 1 character '!' starts at byte position 3 Note that "ő" took 2 bytes, so '!' starts at byte 3. 2.6.6. Switch Go's "switch" is very flexible; you can match on much more than just integers. The cases are evaluated top to bottom until a match is found, and if the "switch" has no expression it switches on "true". It's therefore possible -- and idiomatic -- to write an "if-else-if- else" chain as a "switch". // Convert hexadecimal character to an int value switch { <1> case '0' <= c && c <= '9': <2> return c - '0' <3> case 'a' <= c && c <= 'f': <4> return c - 'a' + 10 case 'A' <= c && c <= 'F': <5> return c - 'A' + 10 } return 0 A "switch" without a condition is the same as "switch true" _1_. We list the different cases. Each "case" statement has a condition that is either true of false. Here _2_ we check if "c" is a number. If "c" is a number we return its value _3_. Check if "c" falls between "a" and "f" _4_. For an "a" we return 10, for "b" we return 11, etc. We also do the same _5_ thing for "A" to "F". Gieben Expires February 26, 2019 [Page 18] Internet-Draft Learning Go August 2018 There is no automatic fall through, you can use "fallthrough" for that. switch i { case 0: fallthrough case 1: <1> f() default: g() <2> "f()" can be called when "i == 0" _1_. With "default" you can specify an action when none of the other cases match. Here "g()" is called when "i" is not 0 or 1 _2_. We could rewrite the above example as: switch i { case 0, 1: <1> f() default: g() You can list cases on one line _1_, separated by commas. 2.7. Built-in Functions A few functions are predefined, meaning you _don't_ have to include any package to get access to them. Table 3 lists them all. +----------+----------+-----------+-----------+ | "close" | "new" | "panic" | "complex" | | "delete" | "make" | "recover" | "real" | | "len" | "append" | "print" | "imag" | | "cap" | "copy" | "println" | | +----------+----------+-----------+-----------+ Table 3: Pre-defined functions in Go. These built-in functions are documented in the "builtin" pseudo package that is included in recent Go releases. Let's go over these functions briefly. close is used in channel communication. It closes a channel. We'll learn more about this in Section 7.3.1. delete is used for deleting entries in maps. len and cap Gieben Expires February 26, 2019 [Page 19] Internet-Draft Learning Go August 2018 are used on a number of different types, "len" is used to return the lengths of strings, maps, slices, and arrays. In the next section Section 2.8.1 we'll look at slices, arrays and the function "cap". new is used for allocating memory for user defined data types. See Section 5.1.1. make is used for allocating memory for built-in types (maps, slices, and channels). See Section 5.1.2. copy, append "copy" is for copying slices. And "append" is for concatenating slices. See Section 2.8.2 in this chapter. panic, recover are used for an _exception_ mechanism. See Section 3.6 for more. print, println are low level printing functions that can be used without reverting to the "fmt" package. These are mainly used for debugging. built-in,println) complex, real, imag all deal with complex numbers. We will not use complex numbers in this book. 2.8. Arrays, Slices, and Maps To store multiple values in a list, you can use arrays, or their more flexible cousin: slices. A dictionary or hash type is also available. It is called a "map" in Go. 2.8.1. Arrays An array is defined by: "[n]", where "n" is the length of the array and "" is the stuff you want to store. To assign or index an element in the array, you use square brackets: var arr [10]int arr[0] = 42 arr[1] = 13 fmt.Printf("The first element is %d\n", arr[0]) Array types like "var arr [10]int" have a fixed size. The size is _part_ of the type. They can't grow, because then they would have a Gieben Expires February 26, 2019 [Page 20] Internet-Draft Learning Go August 2018 different type. Also arrays are values: Assigning one array to another _copies_ all the elements. In particular, if you pass an array to a function it will receive a copy of the array, not a pointer to it. To declare an array you can use the following: "var a [3]int". To initialize it to something other than zero, use a _composite literal_ "a := [3]int{1, 2, 3}". This can be shortened to "a := [...]int{1, 2, 3}", where Go counts the elements automatically. A composite literal allows you to assign a value directly to an array, slice, or map. See Section 5.1.3 for more information. When declaring arrays you _always_ have to type something in between the square brackets, either a number or three dots ("..."), when using a composite literal. When using multidimensional arrays, you can use the following syntax: "a := [2][2]int{ {1,2}, {3,4} }". Now that you know about arrays you will be delighted to learn that you will almost never use them in Go, because there is something much more flexible: slices. 2.8.2. Slices A slice is similar to an array, but it can grow when new elements are added. A slice always refers to an underlying array. What makes slices different from arrays is that a slice is a pointer _to_ an array; slices are reference types. Reference types are created with "make". We detail this further in Section 5. That means that if you assign one slice to another, both refer to the _same_ underlying array. For instance, if a function takes a slice argument, changes it makes to the elements of the slice will be visible to the caller, analogous to passing a pointer to the underlying array. With: "slice := make([]int, 10)", you create a slice which can hold ten elements. Note that the underlying array isn't specified. A slice is always coupled to an array that has a fixed size. For slices we define a capacity and a length The image below shows the creation of an array, then the creation of a slice. First we create an array of "m" elements of the type "int": "var array[m]int" . Next, we create a slice from this array: "slice := array[:n]" . And now we have: o "len(slice) == n" Gieben Expires February 26, 2019 [Page 21] Internet-Draft Learning Go August 2018 o "cap(slice) == m" o "len(array) == cap(array) == m" fig/array-vs-slice.png Array versus slice An array versus a slice. Given an array, or another slice, a new slice is created via "a[n:m]". This creates a new slice which refers to the variable "a", starts at index "n", and ends before index "m". It has length "n - m". a := [...]int{1, 2, 3, 4, 5} <1> s1 := a[2:4] <2> s2 := a[1:5] <3> s3 := a[:] <4> s4 := a[:4] <5> s5 := s2[:] <6> s6 := a[2:4:5] <7> First we define _1_ an array with five elements, from index 0 to 4. From this we create _2_ a slice with the elements from index 2 to 3, this slices contains: "3, 4". Then we we create another slice _3_ from "a": with the elements from index 1 to 4, this contains: "2, 3, 4, 5". With "a[:]" _4_ we create a slice with all the elements in the array. This is a shorthand for: "a[0:len(a)]". And with "a[:4]" _5_ we create a slice with the elements from index 0 to 3, this is short for: "a[0:4]", and gives us a slices that contains: "1, 2, 3, 4". With "s2[:]" we create a slice from the slice "s2" _6_, note that "s5" still refers to the array "a". Finally, we create a slice with the elements from index 3 to 3 _and_ also set the cap to 4 _7_. When working with slices you can overrun the bounds, consider this code. package main func main() { var array [100]int <1> slice := array[0:99] <2> slice[98] = 1 <3> slice[99] = 2 <4> } At _1_ we create an array with a 100 elements, indexed from 0 to 99. Then at _2_ we create a slice that has index 0 to 98. We assign 1 to Gieben Expires February 26, 2019 [Page 22] Internet-Draft Learning Go August 2018 the 99th element _3_ of the slice. This works as expected. But at _4_ we dare to do the impossible, and and try to allocate something beyond the length of the slice and we are greeted with a _runtime_ error: "Error: "throw: index out of range"." If you want to extend a slice, there are a couple of built-in functions that make life easier: "append" and "copy". The append function appends zero or more values to a slice and returns the result: a slice with the same type as the original. If the original slice isn't big enough to fit the added values, append will allocate a new slice that is big enough. So the slice returned by append may refer to a different underlying array than the original slice does. Here's an example: s0 := []int{0, 0} s1 := append(s0, 2) <1> s2 := append(s1, 3, 5, 7) <2> s3 := append(s2, s0...) <3> At _1_ we append a single element, making "s1" equal to "[]int{0, 0, 2}". At _2_ we append multiple elements, making "s2" equal to "[]int{0, 0, 2, 3, 5, 7}". And at _3_ we append a slice, giving us "s3" equal to "[]int{0, 0, 2, 3, 5, 7, 0, 0}". Note the three dots used after "s0..."! This is needed make it clear explicit that you're appending another slice, instead of a single value. The copy function copies slice elements from a source to a destination, and returns the number of elements it copied. This number is the minimum of the length of the source and the length of the destination. For example: var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7} var s = make([]int, 6) n1 := copy(s, a[0:]) <1> n2 := copy(s, s[2:]) <2> After _1_, "n1" is 6, and "s" is "[]int{0, 1, 2, 3, 4, 5}". And after _2_, "n2" is 4, and "s" is "[]int{2, 3, 4, 5, 4, 5}". 2.8.3. Maps Many other languages have a type similar to maps built-in. For instance, Perl has hashes, Python has its dictionaries, and C++ also has maps (as part of the libraries). In Go we have the "map" type. A "map" can be thought of as an array indexed by strings (in its most simple form). Gieben Expires February 26, 2019 [Page 23] Internet-Draft Learning Go August 2018 monthdays := map[string]int{ "Jan": 31, "Feb": 28, "Mar": 31, "Apr": 30, "May": 31, "Jun": 30, "Jul": 31, "Aug": 31, "Sep": 30, "Oct": 31, "Nov": 30, "Dec": 31, <1> } The general syntax for defining a map is "map[]". Here, we define a map that converts from a "string" (month abbreviation) to an "int" (number of days in that month). Note that the trailing comma at _1_ is _required_. Use "make" when only declaring a map: "monthdays := make(map[string]int)". A map is a reference type. For indexing ("searching") the map, we use square brackets. For example, suppose we want to print the number of days in December: "fmt.Printf("%d\n", monthdays["Dec"])" If you are looping over an array, slice, string, or map a, "range" clause will help you again, it returns the key and corresponding value with each invocation. year := 0 for _, days := range monthdays <1> year += days } fmt.Printf("Numbers of days in a year: %d\n", year) At _1_ we use the underscore to ignore (assign to nothing) the key returned by "range". We are only interested in the values from "monthdays". To add elements to the map, you would add new month with: "monthdays["Undecim"] = 30". If you use a key that already exists, the value will be silently overwritten: "monthdays["Feb"] = 29". To test for existence you would use the following: "value, present := monthdays["Jan"]". If the key "Jan" exists, "present" will be true. It's more Go like to name "present" "ok", and use: "v, ok := monthdays["Jan"]". In Go we call this the "comma ok" form. You can remove elements from the "map": "delete(monthdays, "Mar")" . In general the syntax "delete(m, x)" will delete the map entry retrieved by the expression "m[x]". Gieben Expires February 26, 2019 [Page 24] Internet-Draft Learning Go August 2018 2.9. Exercises 2.9.1. For-loop 1. Create a loop with the "for" construct. Make it loop 10 times and print out the loop counter with the "fmt" package. 2. Rewrite the loop from 1 to use "goto". The keyword "for" may not be used. 3. Rewrite the loop again so that it fills an array and then prints that array to the screen. 2.9.2. Answer 1. There are many possibilities. One solution could be: package main import "fmt" func main() { for i := 0; i < 10; i++ { fmt.Println("%d", i) } } Let's compile this and look at the output. % go build for.go % ./for 0 1 . . . 9 1. Rewriting the loop results in code that should look something like this (only showing the "main"-function): Gieben Expires February 26, 2019 [Page 25] Internet-Draft Learning Go August 2018 func main() { i := 0 <1> Loop: <2> if i < 10 { fmt.Printf("%d\n", i) i++ goto Loop <3> } } At _1_ we define our loop variable. And at _2_ we define a label and at _3_ we jump to this label. 2. The following is one possible solution: package main import "fmt" func main() { var arr [10]int <1> for i := 0; i < 10; i++ { arr[i] = i <2> } fmt.Printf("%v", arr) <3> } Here _1_ we create an array with 10 elements. Which we then fill _2_ one by one. And finally we print it _3_ with "%v" which lets Go to print the value for us. You could even do this in one fell swoop by using a composite literal: fmt.Printf("%v\n", [...]int{0,1,2,3,4,5,6,7,8,9}) 2.9.3. Average 1. Write code to calculate the average of a "float64" slice. In a later exercise you will make it into a function. 2.9.4. Answer 1. The following code calculates the average. Gieben Expires February 26, 2019 [Page 26] Internet-Draft Learning Go August 2018 sum := 0.0 switch len(xs) { case 0: <1> avg = 0 default: <2> for _, v := range xs { sum += v } avg = sum / float64(len(xs)) <3> } Here at _1_ we check if the length is zero and if so, we return 0. Otherwise we calculate the average at _2_. We have to convert the value return from "len" to a "float64" to make the division work at _3_. 2.9.5. FizzBuzz 1. Solve this problem, called the Fizz-Buzz [fizzbuzz] problem: Write a program that prints the numbers from 1 to 100. But for multiples of three print, "Fizz" instead of the number, and for multiples of five, print "Buzz". For numbers which are multiples of both three and five, print "FizzBuzz". 2.9.6. Answer 1. A possible solution to this problem is the following program. Gieben Expires February 26, 2019 [Page 27] Internet-Draft Learning Go August 2018 package main import "fmt" func main() { const ( FIZZ = 3 <1> BUZZ = 5 ) var p bool <2> for i := 1; i < 100; i++ { <3> p = false if i%FIZZ == 0 { <4> fmt.Printf("Fizz") p = true } if i%BUZZ == 0 { <5> fmt.Printf("Buzz") p = true } if !p { <6> fmt.Printf("%v", i) } fmt.Println() } } Here _1_ we define two constants to make our code more readable, see Section 2.3.3. At _2_ we define a boolean that keeps track if we already printed something. At _3_ we start our for-loop, see Section 2.6.3. If the value is divisible by FIZZ - that is, 3 - , we print "Fizz" _4_. And at _5_ we check if the value is divisble by BUZZ -- that is, 5 -- if so print "Buzz". Note that we have also taken care of the FizzBuzz case. At _6_, if printed neither Fizz nor Buzz printed, we print the value. 3. Functions I'm always delighted by the light touch and stillness of early programming languages. Not much text; a lot gets done. Old programs read like quiet conversations between a well-spoken research worker and a well- studied mechanical colleague, not as a debate with a compiler. Who'd have guessed sophistication bought such noise? ------------------------------------------------------------ Richard P. Gabriel Gieben Expires February 26, 2019 [Page 28] Internet-Draft Learning Go August 2018 Functions are the basic building blocks of Go programs; all interesting stuff happens in them. Here is an example of how you can declare a function: type mytype int func (p mytype) funcname(q int) (r,s int) { return 0,0 } <1> <2> <3> <4> <5> <6> To declare a function, you use the "func" keyword _1_. You can optionally bind _2_ to a specific type called receiver (a function with a receiver is usually called a method). This will be explored in Section 6. Next _3_ you write the name of your function. Here _4_ we define that the variable "q" of type "int" is the input parameter. Parameters are passed _pass-by-value_. The variables "r" and "s" _5_ are the _named return parameters_ (((functions, named return parameters))) for this function. Functions in Go can have multiple return values. This is very useful to return a value _and_ error. This removes the need for in-band error returns (such as -1 for "EOF") and modifying an argument. If you want the return parameters not to be named you only give the types: "(int, int)". If you have only one value to return you may omit the parentheses. If your function is a subroutine and does not have anything to return you may omit this entirely. Finally, we have the body _6_ of the function. Note that "return" is a statement so the braces around the parameter(s) are optional. As said the return or result parameters of a Go function can be given names and used as regular variables, just like the incoming parameters. When named, they are initialized to the zero values for their types when the function begins. If the function executes a "return" statement with no arguments, the current values of the result parameters are returned. Using these features enables you (again) to do more with less code. The names are not mandatory but they can make code shorter and clearer: _they are documentation_. However don't overuse this feature, especially in longer functions where it might not be immediately apparent what is returned. Functions can be declared in any order you wish. The compiler scans the entire file before execution, so function prototyping is a thing of the past in Go. Go does not allow nested functions, but you can work around this with anonymous functions. See the Section Section 3.2 in this chapter. Recursive functions work just as in other languages: Gieben Expires February 26, 2019 [Page 29] Internet-Draft Learning Go August 2018 func rec(i int) { if i == 10 { <1> return } rec(i+1) <2> fmt.Printf("%d ", i) } Here _2_ we call the same function again, "rec" returns when "i" has the value 10, this is checked on the second line _1_. This function prints: "9 8 7 6 5 4 3 2 1 0", when called as "rec(0)". 3.1. Scope Variables declared outside any functions are _global_ in Go, those defined in functions are _local_ to those functions. If names overlap - a local variable is declared with the same name as a global one - the local variable hides the global one when the current function is executed. In the following example we call "g()" from "f()": package main var a int <1> func main() { a = 5 print(a) f() } func f() { a := 6 <2> print(a) g() } func g() { print(a) } Here _1_, we declare "a" to be a global variable of type "int". Then in the "main" function we give the _global_ "a" the value of 5, after printing it we call the function "f". Then here _2_, "a := 6", we create a _new, local_ variable also called "a". This new "a" gets the value of 6, which we then print. Then we call "g", which uses the _global_ "a" again and prints "a"'s value set in "main". Thus Gieben Expires February 26, 2019 [Page 30] Internet-Draft Learning Go August 2018 the output will be: "565". A _local_ variable is _only_ valid when we are executing the function in which it is defined. Note that the ":=" used in line 12 is sometimes hard to spot so it is generally advised _not_ to use the same name for global and local variables. 3.2. Functions as values As with almost everything in Go, functions are also _just_ values. They can be assigned to variables as follows: import "fmt" func main() { a := func() { <1> fmt.Println("Hello") } <2> a() <3> } "a" is defined as an anonymous (nameless) function _1_. Note the lack of parentheses "()" after "a". If there were, that would be to _call_ some function with the name "a" before we have defined what "a" is. Once "a" is defined, then we can _call_ it, _3_. Functions--as--values may be used in other places, for example maps. Here we convert from integers to functions: var xs = map[int]func() int{ 1: func() int { return 10 }, 2: func() int { return 20 }, 3: func() int { return 30 }, } Note that the final comma on second to last line is _mandatory_. Or you can write a function that takes a function as its parameter, for example a "Map" function that works on "int" slices. This is left as an exercise for the reader; see the exercise Section 3.7.15. 3.3. Callbacks Because functions are values they are easy to pass to functions, from where they can be used as callbacks. First define a function that does "something" with an integer value: func printit(x int) { fmt.Printf("%v\n", x) } Gieben Expires February 26, 2019 [Page 31] Internet-Draft Learning Go August 2018 This function does not return a value and just prints its argument. The _signature_ of this function is: "func printit(int)", or without the function name: "func(int)". To create a new function that uses this one as a callback we need to use this signature: func callback(y int, f func(int)) { f(y) } Here we create a new function that takes two parameters: "y int", i.e. just an "int" and "f func(int)", i.e. a function that takes an int and returns nothing. The parameter "f" is the variable holding that function. It can be used as any other function, and we execute the function on line 2 with the parameter "y": "f(y)" 3.4. Deferred Code Suppose you have a function in which you open a file and perform various writes and reads on it. In such a function there are often spots where you want to return early. If you do that, you will need to close the file descriptor you are working on. This often leads to the following code: func ReadWrite() bool { file.Open("file") // Do your thing if failureX { file.Close() <1> return false } if failureY { file.Close() <1> return false } file.Close() <1> return true <2> } Note that we repeat a lot of code here; you can see the that "file.Close()" is called at _1_. To overcome this, Go has the "defer" keyword. After "defer" you specify a function which is called just _before_ _2_ the current function exits. With "defer" we can rewrite the above code as follows. It makes the function more readable and it puts the "Close" _right next_ to the "Open". Gieben Expires February 26, 2019 [Page 32] Internet-Draft Learning Go August 2018 func ReadWrite() bool { file.Open("filename") defer file.Close() <1> // Do your thing if failureX { return false <2> } if failureY { return false <2> } return true <2> } At _1_ "file.Close()" is added to the defer list. "Close" is now done automatically at _2_. This makes the function shorter and more readable. It puts the "Close" right next to the "Open". You can put multiple functions on the "defer list", like this example from for i := 0; i < 5; i++ { defer fmt.Printf("%d ", i) } Deferred functions are executed in LIFO order, so the above code prints: "4 3 2 1 0". With "defer" you can even change return values, provided that you are using named result parameters and a function literal , i.e: defer func() {/* ... */}() Here we use a function without a name and specify the body of the function inline, basically we're creating a nameless function on the spot. The final braces are needed because "defer" needs a function call, not a function value. If our anonymous function would take an parameter it would be easier to see why we need the braces: defer func(x int) {/* ... */}(5) In this (unnamed) function you can access any named return parameter: func f() (ret int) defer func() { <1> ret++ }() return 0 } Gieben Expires February 26, 2019 [Page 33] Internet-Draft Learning Go August 2018 Here _1_ we specify our function, the named return value "ret" is initialized with zero. The nameless function in the defer increments the value of "ret" with 1. The "return 0" on line 5 _will not be the returned value_, because of "defer". The function "f" will return 1! 3.5. Variadic Parameter Functions that take a variable number of parameters are known as variadic functions. To declare a function as variadic, do something like this: func myfunc(arg ...int) {} The "arg ...int" instructs Go to see this as a function that takes a variable number of arguments. Note that these arguments all have to have the type "int". In the body of your function the variable "arg" is a slice of ints: for _, n := range arg { fmt.Printf("And the number is: %d\n", n) } We range over the arguments on the first line. We are not interested in the index as returned by "range", hence the use of the underscore there. In the body of the "range" we just print the parameters we were given. If you don't specify the type of the variadic argument it defaults to the empty interface "interface{}" (see Chapter Section 6). Suppose we have another variadic function called "myfunc2", the following example shows how to pass variadic arguments to it: func myfunc(arg ...int) { myfunc2(arg...) myfunc2(arg[:2]...) } With "myfunc2(arg...)" we pass all the parameters to "myfunc2", but because the variadic parameters is just a slice, we can use some slice tricks as well. 3.6. Panic and recovering Go does not have an exception mechanism: you cannot throw exceptions. Instead it uses a panic-and-recover mechanism. It is worth remembering that you should use this as a last resort, your code will not look, or be, better if it is littered with panics. It's a Gieben Expires February 26, 2019 [Page 34] Internet-Draft Learning Go August 2018 powerful tool: use it wisely. So, how do you use it? In the words of the Go Authors [go_blog_panic]: Panic is a built-in function that stops the ordinary flow of control and begins panicking. When the function "F" calls "panic", execution of "F" stops, any deferred functions in "F" are executed normally, and then "F" returns to its caller. To the caller, "F" then behaves like a call to "panic". The process continues up the stack until all functions in the current goroutine have returned, at which point the program crashes. Panics can be initiated by invoking "panic" directly. They can also be caused by _runtime errors_, such as out-of-bounds array accesses. Recover is a built-in function that regains control of a panicking goroutine. Recover is _only_ useful inside _deferred_ functions. During normal execution, a call to "recover" will return "nil" and have no other effect. If the current goroutine is panicking, a call to "recover" will capture the value given to "panic" and resume normal execution. This function checks if the function it gets as argument will panic when it is executed: func Panic(f func()) (b bool) { <1> defer func() { <2> if x := recover(); x != nil { b = true } }() f() <3> return <4> } We define a new function "Panic" _1_ that takes a function as an argument (see Section 3.2). It returns true if "f" panics when run, else false. We then _2_ define a "defer" function that utilizes "recover". If the current goroutine panics, this defer function will notice that. If "recover()" returns non-"nil" we set "b" to true. At _3_ Execute the function we received as the argument. And finally _4_ we return the value of "b". Because "b" is a named return parameter. The following code fragment, shows how we can use this function: Gieben Expires February 26, 2019 [Page 35] Internet-Draft Learning Go August 2018 func panicy() { var a []int a[3] = 5 } func main() { fmt.Println(Panic(panicy)) } On line 3 the "a[3] = 5" triggers a _runtime_ out of bounds error which results in a panic. Thus this program will print "true". If we change line 2: "var a []int" to "var a [4]int" the function "panicy" does not panic anymore. Why? 3.7. Exercises 3.7.1. Average 1. Write a function that calculates the average of a "float64" slice. 3.7.2. Answer 1. The following function calculates the average: package main func average(xs []float64) (avg float64) { //<1> sum := 0.0 switch len(xs) { case 0: //<2> avg = 0 default: //<3> for _, v := range xs { sum += v } avg = sum / float64(len(xs)) //<4> } return //<5> } At _1_ we use a named return parameter. If the length of "xs" is zero _2_, we return 0. Otherwise _3_, we calculate the average. At _4_ we convert the value to a "float64" to make the division work as "len" returns an "int". Finally, at _5_ we reutrn our avarage. Gieben Expires February 26, 2019 [Page 36] Internet-Draft Learning Go August 2018 3.7.3. Bubble sort 1. Write a function that performs a bubble sort on a slice of ints. From [bubblesort]: It works by repeatedly stepping through the list to be sorted, comparing each pair of adjacent items and swapping them if they are in the wrong order. The pass through the list is repeated until no swaps are needed, which indicates that the list is sorted. The algorithm gets its name from the way smaller elements "bubble" to the top of the list. It also gives an example in pseudo code: procedure bubbleSort( A : list of sortable items ) do swapped = false for each i in 1 to length(A) - 1 inclusive do: if A[i-1] > A[i] then swap( A[i-1], A[i] ) swapped = true end if end for while swapped end procedure 3.7.4. Answer 1. Bubble sort isn't terribly efficient. For "n" elements it scales "O(n^2)". But bubble sort is easy to implement: func main() { n := []int{5, -1, 0, 12, 3, 5} fmt.Printf("unsorted %v\n", n) bubblesort(n) fmt.Printf("sorted %v\n", n) } func bubblesort(n []int) { for i := 0; i < len(n)-1; i++ { for j := i + 1; j < len(n); j++ { if n[j] < n[i] { n[i], n[j] = n[j], n[i] } Because a slice is a reference type, the "bubblesort" function works and does not need to return a sorted slice. Gieben Expires February 26, 2019 [Page 37] Internet-Draft Learning Go August 2018 3.7.5. For-loop II 1. Take what you did in exercise to write the for loop and extend it a bit. Put the body of the for loop - the "fmt.Printf" - in a separate function. 3.7.6. Answer 1. <{{src/for-func.go}} 3.7.7. Fibonacci 1. The Fibonacci sequence starts as follows: "1, 1, 2, 3, 5, 8, 13, \ldots" Or in mathematical terms: "x_1 = 1; x_2 = 1; x_n = x_{n-1} + x_{n-2}\quad\forall n > 2". Write a function that takes an "int" value and gives that many terms of the Fibonacci sequence. 3.7.8. Answer 1. The following program calculates Fibonacci numbers: package main import "fmt" func fibonacci(value int) []int { x := make([]int, value) <1> x[0], x[1] = 1, 1 <2> for n := 2; n < value; n++ { x[n] = x[n-1] + x[n-2] <3> } return x <4> } func main() { for _, term := range fibonacci(10) { <5> fmt.Printf("%v ", term) } } At _1_ we create an array to hold the integers up to the value given in the function call. At _2_ we start the Fibonacci calculation. Then _3_: "x_n = x_{n-1} + x_{n-2}". At _4_ we return the _entire_ array. And at _5_ we use the "range" keyword to "walk" the numbers returned by the Fibonacci function. Here up to 10. Finally, we print the numbers. Gieben Expires February 26, 2019 [Page 38] Internet-Draft Learning Go August 2018 3.7.9. Var args 1. Write a function that takes a variable number of ints and print each integer on a separate line. 3.7.10. Answer 1. For this we need the "{...}"-syntax to signal we define a function that takes an arbitrary number of arguments. package main import "fmt" func main() { printthem(1, 4, 5, 7, 4) printthem(1, 2, 4) } func printthem(numbers ...int) { for _, d := range numbers { fmt.Printf("%d\n", d) } } 3.7.11. Functions that return functions 1. Write a function that returns a function that performs a "+2" on integers. Name the function "plusTwo". You should then be able do the following: p := plusTwo() fmt.Printf("%v\n", p(2)) Which should print 4. See Section 3.3. 2. Generalize the function from above and create a "plusX(x)" which returns functions that add "x" to an integer. 3.7.12. Answer 1. Define a new function that returns a function: "return func(x int) int { return x + 2 }" Function literals at work, we define the +2--function right there in the return statement. Gieben Expires February 26, 2019 [Page 39] Internet-Draft Learning Go August 2018 func main() { p2 := plusTwo() fmt.Printf("%v\n",p2(2)) } func plusTwo() func(int) int { <1> return func(x int) int { return x + 2 } <2> } 2. Here we use a closure: func plusX(x int) func(int) int { <1> return func(y int) int { return x + y } <2> } Here _1_, we again define a function that returns a function. We use the _local_ variable "x" in the function literal at _2_. 3.7.13. Maximum 1. Write a function that finds the maximum value in an "int" slice ("[]int"). 3.7.14. Answer 1. This function returns the largest int in the slice \var{l}: func max(l []int) (max int) { <1> max = l[0] for _, v := range l { <2> if v > max { <3> max = v } } return <4> } At _1_ we use a named return parameter. At _2_ we loop over "l". The index of the element is not important. At _3_, if we find a new maximum, we remember it. And at _4_ we have a "lone" return; the current value of "max" is now returned. 3.7.15. Map function A "map()"-function is a function that takes a function and a list. The function is applied to each member in the list and a new list containing these calculated values is returned. Thus: Gieben Expires February 26, 2019 [Page 40] Internet-Draft Learning Go August 2018 "\mathrm{map}(f(), (a_1,a_2,\ldots,a_{n-1},a_n)) = (f(a_1), f(a_2),\ldots,f(a_{n-1}), f(a_n)) " 1. Write a simple "map()"-function in Go. It is sufficient for this function only to work for ints. 3.7.16. Answer 1. A possible answer: func Map(f func(int) int, l []int) []int { j := make([]int, len(l)) for k, v := range l { j[k] = f(v) } return j } func main() { m := []int{1, 3, 4} f := func(i int) int { return i * i } fmt.Printf("%v", (Map(f, m))) } 3.7.17. Stack 1. Create a simple stack which can hold a fixed number of ints. It does not have to grow beyond this limit. Define "push" -- put something on the stack -- and "pop" -- retrieve something from the stack -- functions. The stack should be a LIFO (last in, first out) stack. fig/stack.png A stack A stack. 1. Write a "String" method which converts the stack to a string representation. The stack in the figure could be represented as: "[0:m] [1:l] [2:k]" . 3.7.18. Answer 1. First we define a new type that represents a stack; we need an array (to hold the keys) and an index, which points to the last element. Our small stack can only hold 10 elements. Gieben Expires February 26, 2019 [Page 41] Internet-Draft Learning Go August 2018 type stack struct { i int data [10]int } Next we need the "push" and "pop" functions to actually use the thing. First we show the _wrong_ solution! In Go, data passed to functions is _passed-by-value_ meaning a copy is created and given to the function. The first stab for the function "push" could be: func (s stack) push(k int) { if s.i+1 > 9 { return } s.data[s.i] = k s.i++ } The function works on the "s" which is of the type "stack". To use this we just call "s.push(50)", to push the integer 50 on the stack. But the push function gets a copy of "s", so it is _not_ working on the _real_ thing. Nothing gets pushed to our stack. For example the following code: var s stack s.push(25) fmt.Printf("stack %v\n", s); s.push(14) fmt.Printf("stack %v\n", s); prints: stack [0:0] stack [0:0] To solve this we need to give the function "push" a pointer to the stack. This means we need to change "push" from func (s stack) push(k int) to func (s *stack) push(k int). Gieben Expires February 26, 2019 [Page 42] Internet-Draft Learning Go August 2018 We should now use "new()" (see Section 5.1.1). in Section 5 to create a _pointer_ to a newly allocated "stack", so line 1 from the example above needs to be "s := new(stack)" . And our two functions become: func (s *stack) push(k int) { s.data[s.i] = k s.i++ } func (s *stack) pop() int { s.i-- ret := s.data[s.i] s.data[s.i] = 0 return ret } Which we then use as follows: func main() { var s stack s.push(25) s.push(14) fmt.Printf("stack %v\n", s) } 1. "fmt.Printf("%v")" can print any value ("%v") that satisfies the "Stringer" interface (see Section 6). For this to work we only need to define a "String()" function for our type: func (s stack) String() string { var str string for i := 0; i <= s.i; i++ { str = str + "[" + strconv.Itoa(i) + ":" + strconv.Itoa(s.data[i]) + "]" } return str } 4. Packages "^(") ------------------------------------------------------------ Answer to whether there is a bit wise negation operator -- Ken Thompson Gieben Expires February 26, 2019 [Page 43] Internet-Draft Learning Go August 2018 A package is a collection of functions and data. You declare a package with the "package" keyword. The filename does not have to match the package name. The convention for package names is to use lowercase characters. Go packages may consist of multiple files, but they share the "package " line. Let's define a package "even" in the file "even.go". package even <1> func Even(i int) bool { <2> return i%2 == 0 } func odd(i int) bool { <3> return i%2 == 1 } Here _1_ we start a new namespace: "even". The function "Even" _2_ starts with a capital letter. This means the function is _exported_, and may be used outside our package (more on that later). The function "odd" _3_ does not start with a capital letter, so it is a _private_ function. Now we just need to build the package. We create a directory under "$GOPATH", and copy "even.go" there (see Section 2.2 in Section 2). % mkdir $GOPATH/src/even % cp even.go $GOPATH/src/even % go build % go install Now we can use the package in our own program "myeven.go": package main import ( <1> "even" <2> "fmt" <3> ) func main() { i := 5 fmt.Printf("Is %d even? %v\n", i, even.Even(i)) <4> } Import _1_ the following packages. The _local_ package "even" is imported here _2_. This _3_ imports the official "fmt" package. And Gieben Expires February 26, 2019 [Page 44] Internet-Draft Learning Go August 2018 now we use _4_ the function from the "even" package. The syntax for accessing a function from a package is ".FunctionName()". And finally we can build our program. % go build myeven.go % ./myeven Is 5 even? false If we change our "myeven.go" at _4_ to use the unexported function "even.odd": "fmt.Printf("Is %d even? %v\n", i, even.odd(i))" We get an error when compiling, because we are trying to use a _private_ function: myeven.go: cannot refer to unexported name even.odd Note that the "starts with capital "\rightarrow" exported", "starts with lower-case "\rightarrow" private" rule also extends to other names (new types, global variables) defined in the package. Note that the term "capital" is not limited to US-ASCII -- it extends to all bicameral alphabets (Latin, Greek, Cyrillic, Armenian and Coptic). 4.1. Identifiers The Go standard library names some function with the old (Unix) names while others are in CamelCase. The convention is to leave well-known legacy not-quite-words alone rather than try to figure out where the capital letters go: "Atoi", "Getwd", "Chmod". CamelCasing works best when you have whole words to work with: "ReadFile", "NewWriter", "MakeSlice". The convention in Go is to use CamelCase rather than underscores to write multi-word names. As we did above in our "myeven" program, accessing content from an imported (with "import" ) package is done with using the package's name and then a dot. After "import "bytes"" the importing program can talk about "bytes.Buffer". A package name should be good, short, concise and evocative. The convention in Go is that package names are lowercase, single word names. The package name used in the "import" statement is the default name used. But if the need arises (two different packages with the same name for instance), you can override this default: "import bar "bytes"" The function "Buffer" is now accessed as "bar.Buffer". Another convention is that the package name is the base name of its source directory; the package in "src/compress/gzip" is imported as "compress/gzip" but has name "gzip", not "compress/gzip". Gieben Expires February 26, 2019 [Page 45] Internet-Draft Learning Go August 2018 It is important to avoid stuttering when naming things. For instance, the buffered reader type in the "bufio" package is called "Reader", not "BufReader", because users see it as "bufio.Reader", which is a clear, concise name. Similarly, the function to make new instances of "ring.Ring" (package "container/ring"), would normally be called "NewRing", but since "Ring" is the only type exported by the package, and since the package is called "ring", it's called just "New". Clients of the package see that as "ring.New". Use the package structure to help you choose good names. Another short example is "once.Do" (see package "sync"); "once.Do(setup)" reads well and would not be improved by writing "once.DoOrWaitUntilDone(setup)". Long names don't automatically make things more readable. 4.2. Documenting packages When we created our "even" package, we skipped over an important item: documentation. Each package should have a _package comment_, a block comment preceding the "package" clause. In our case we should extend the beginning of the package, with: // The even package implements a fast function for detecting if an integer // is even or not. package even When running "go doc" this will show up at the top of the page. When a package consists of multiple files the package comment should only appear in one file. A common convention (in really big packages) is to have a separate "doc.go" that only holds the package comment. Here is a snippet from the official "regexp" package: /* The regexp package implements a simple library for regular expressions. The syntax of the regular expressions accepted is: regexp: concatenation { '|' concatenation } */ package regexp Each defined (and exported) function should have a small line of text documenting the behavior of the function. Again to extend our "even" package: Gieben Expires February 26, 2019 [Page 46] Internet-Draft Learning Go August 2018 // Even returns true of i is even. Otherwise false is returned. func Even(i int) bool { And even though "odd" is not exported, it's good form to document it as well. // odd is the opposite of Even. func odd(i int) bool { 4.3. Testing packages In Go it is customary to write (unit) tests for your package. Writing tests involves the "testing" package and the program "go test". Both have excellent documentation. The "go test" program runs all the test functions. Without any defined tests for our "even" package, "go test" yields: % go test ? even [no test files] Let us fix this by defining a test in a test file. Test files reside in the package directory and are named "*_test.go". Those test files are just like other Go programs, but "go test" will only execute the test functions. Each test function has the same signature and its name should start with "Test": "func TestXxx(t *testing.T)" . When writing test you will need to tell "go test" whether a test was successful or not. A successful test function just returns. When the test fails you can signal this with the following functions. These are the most important ones (see "go doc testing" or "go help testfunc" for more): o "func (t *T) Fail()", "Fail" marks the test function as having failed but continues execution. o "func (t *T) FailNow()", "FailNow" marks the test function as having failed and stops its execution. Any remaining tests in this file are skipped, and execution continues with the next test. o "func (t *T) Log(args ...interface{})", "Log" formats its arguments using default formatting, analogous to "Print()", and records the text in the error log. o "func (t *T) Fatal(args ...interface{})", "Fatal" is equivalent to "Log()" followed by "FailNow()". Gieben Expires February 26, 2019 [Page 47] Internet-Draft Learning Go August 2018 Putting all this together we can write our test. First we pick a name: "even_test.go". Then we add the following contents: package even <1> import "testing" <2> func TestEven(t *testing.T) { <3> if !Even(2) { t.Log("2 should be even!") t.Fail() } } A test file belongs to the current _1_ package. This is not only convenient, but also allows tests of unexported functions and structures. We then _2_ import the "testing" package. And finally the test we want to execute. The code here _3_ should hold no surprises: we check if the "Even" function works OK. And now, the moment we have been waiting for executing the test. % go test ok even 0.001s Our test ran and reported "ok". Success! If we redefine our test function, we can see the result of a failed test: // Entering the twilight zone func TestEven(t *testing.T) { if Even(2) { t.Log("2 should be odd!") t.Fail() } } We now get: FAIL even 0.004s --- FAIL: TestEven (0.00 seconds) 2 should be odd! FAIL And you can act accordingly (by fixing the test for instance). Writing new packages should go hand in hand with writing (some) documentation and test functions. It will make your code better and it shows that you really put in the effort. Gieben Expires February 26, 2019 [Page 48] Internet-Draft Learning Go August 2018 The Go test suite also allows you to incorporate example functions which serve as documentation _and_ as tests. These functions need to start with "Example". func ExampleEven() { if Even(2) { fmt.Printf("Is even\n") } // Output: <1> // Is even } Those last two comments lines _1_ are part of the example, "go test" uses those to check the _generated_ output with the text in the comments. If there is a mismatch the test fails. 4.4. Useful packages The standard libary of Go includes a huge number of packages. It is very enlightening to browse the "$GOROOT/src" directory and look at the packages. We cannot comment on each package, but the following are worth a mention: fmt Package "fmt" implements formatted I/O with functions analogous to C's "printf" and "scanf". The format verbs are derived from C's but are simpler. Some verbs (%-sequences) that can be used: * _%v_, the value in a default format. when printing structs, the plus flag (%+v) adds field names. * _%#v_, a Go-syntax representation of the value. * _%T_, a Go-syntax representation of the type of the value. io This package provides basic interfaces to I/O primitives. Its primary job is to wrap existing implementations of such primitives, such as those in package os, into shared public interfaces that abstract the functionality, plus some other related primitives. bufio This package implements buffered I/O. It wraps an "io.Reader" or "io.Writer" object, creating another object (Reader or Writer) that also implements the interface but provides buffering and some help for textual I/O. Gieben Expires February 26, 2019 [Page 49] Internet-Draft Learning Go August 2018 sort The "sort" package provides primitives for sorting arrays and user-defined collections. strconv The "strconv" package implements conversions to and from string representations of basic data types. os The "os" package provides a platform-independent interface to operating system functionality. The design is Unix-like. sync The package "sync" provides basic synchronization primitives such as mutual exclusion locks. flag The "flag" package implements command-line flag parsing. encoding/json The "encoding/json" package implements encoding and decoding of JSON objects as defined in RFC 4627 [RFC4627]. html/template Data-driven templates for generating textual output such as HTML. Templates are executed by applying them to a data structure. Annotations in the template refer to elements of the data structure (typically a field of a struct or a key in a map) to control execution and derive values to be displayed. The template walks the structure as it executes and the "cursor" @ represents the value at the current location in the structure. net/http The "net/http" package implements parsing of HTTP requests, replies, and URLs and provides an extensible HTTP server and a basic HTTP client. unsafe The "unsafe" package contains operations that step around the type safety of Go programs. Normally you don't need this package, but it is worth mentioning that _unsafe_ Go programs are possible. reflect The "reflect" package implements run-time reflection, allowing a program to manipulate objects with arbitrary types. The typical use is to take a value with static type "interface{}" and extract its dynamic type information by calling "TypeOf", which returns an Gieben Expires February 26, 2019 [Page 50] Internet-Draft Learning Go August 2018 object with interface type "Type". See Section 6, Section Section 6.8. os/exec The "os/exec" package runs external commands. 4.5. Exercises 4.5.1. Stack as package 1. See the Stack exercise. In this exercise we want to create a separate package for that code. Create a proper package for your stack implementation, "Push", "Pop" and the "Stack" type need to be exported. 2. Write a simple unit test for this package. You should at least test that a "Pop" works after a "Push". 4.5.2. Answer 1. There are a few details that should be changed to make a proper package for our stack. First, the exported functions should begin with a capital letter and so should "Stack". The package file is named "stack-as-package.go" and contains: package stack // Stack holds the items. type Stack struct { i int data [10]int } // Push pushes an item on the stack. func (s *Stack) Push(k int) { s.data[s.i] = k s.i++ } // Pop pops an item from the stack. func (s *Stack) Pop() (ret int) { s.i-- ret = s.data[s.i] return } 2. To make the unit testing work properly you need to do some preparations. We'll come to those in a minute. First the actual Gieben Expires February 26, 2019 [Page 51] Internet-Draft Learning Go August 2018 unit test. Create a file with the name "pushpop_test.go", with the following contents: package stack import "testing" func TestPushPop(t *testing.T) { c := new(Stack) c.Push(5) if c.Pop() != 5 { t.Log("Pop doesn't give 5") t.Fail() } } For "go test" to work we need to put our package files in a directory under "$GOPATH/src": % mkdir $GOPATH/src/stack % cp pushpop_test.go $GOPATH/src/stack % cp stack-as-package.go $GOPATH/src/stack Yields: % go test stack ok stack 0.001s 4.5.3. Calculator 1. Create a reverse polish calculator. Use your stack package. 4.5.4. Answer 1. This is one answer: package main import ( "bufio" "fmt" "os" "strconv" ) var reader *bufio.Reader = bufio.NewReader(os.Stdin) var st = new(Stack) Gieben Expires February 26, 2019 [Page 52] Internet-Draft Learning Go August 2018 type Stack struct { i int data [10]int } func (s *Stack) push(k int) { if s.i+1 > 9 { return } s.data[s.i] = k s.i++ } func (s *Stack) pop() (ret int) { s.i-- if s.i < 0 { s.i = 0 return } ret = s.data[s.i] return } func main() { for { s, err := reader.ReadString('\n') var token string if err != nil { return } for _, c := range s { switch { case c >= '0' && c <= '9': token = token + string(c) case c == ' ': r, _ := strconv.Atoi(token) st.push(r) token = "" case c == '+': fmt.Printf("%d\n", st.pop()+st.pop()) case c == '*': fmt.Printf("%d\n", st.pop()*st.pop()) case c == '-': p := st.pop() q := st.pop() fmt.Printf("%d\n", q-p) case c == 'q': return Gieben Expires February 26, 2019 [Page 53] Internet-Draft Learning Go August 2018 default: //error } } } } 5. Beyond the basics Go has pointers but not pointer arithmetic. You cannot use a pointer variable to walk through the bytes of a string. ------------------------------------------------------------ Go For C++ Programmers -- Go Authors In this chapter we delve deeper into the language. Go has pointers. There is however no pointer arithmetic, so they act more like references than pointers that you may know from C. Pointers are useful. Remember that when you call a function in Go, the variables are _pass-by-value_. So, for efficiency and the possibility to modify a passed value _in_ functions we have pointers. You declare a pointer by prefixing the type with an '"*"': "var p *int". Now "p" is a pointer to an integer value. All newly declared variables are assigned their zero value and pointers are no different. A newly declared pointer, or just a pointer that points to nothing, has a nil-value . In other languages this is often called a NULL pointer in Go it is just "nil". To make a pointer point to something you can use the address-of operator ("&"), which we demonstrate here: var p *int fmt.Printf("%v", p) <1> var i int <2> p = &i <3> fmt.Printf("%v", p) <4> This _1_ Prints "nil". Declare _2_ an integer variable "i". Make "p" point _3_ to "i", i.e. take the address of "i". And this _4_ will print something like "0x7ff96b81c000a". De-referencing a pointer is done by prefixing the pointer variable with "*". Gieben Expires February 26, 2019 [Page 54] Internet-Draft Learning Go August 2018 As said, there is no pointer arithmetic, so if you write: "*p++", it is interpreted as "(*p)++": first reference and then increment the value. 5.1. Allocation Go also has garbage collection, meaning that you don't have to worry about memory deallocation. To allocate memory Go has two primitives, "new" and "make". They do different things and apply to different types, which can be confusing, but the rules are simple. The following sections show how to handle allocation in Go and hopefully clarifies the somewhat artificial distinction between "new" and "make" . 5.1.1. Allocation with new The built-in function "new" is essentially the same as its namesakes in other languages: "new(T)" allocates zeroed storage for a new item of type "T" and returns its address, a value of type "*T". Or in other words, it returns a pointer to a newly allocated zero value of type "T". This is important to remember. The documentation for "bytes.Buffer" states that "the zero value for Buffer is an empty buffer ready to use.". Similarly, "sync.Mutex" does not have an explicit constructor or Init method. Instead, the zero value for a "sync.Mutex" is defined to be an unlocked mutex. 5.1.2. Allocation with make The built-in function "make(T, args)" serves a purpose different from "new(T)". It creates slices, maps, and channels _only_, and it returns an initialized (not zero!) value of type "T", and not a pointer: "*T". The reason for the distinction is that these three types are, under the covers, references to data structures that must be initialized before use. A slice, for example, is a three-item descriptor containing a pointer to the data (inside an array), the length, and the capacity; until those items are initialized, the slice is "nil". For slices, maps, and channels, "make" initializes the internal data structure and prepares the value for use. For instance, "make([]int, 10, 100)" allocates an array of 100 ints and then creates a slice structure with length 10 and a capacity of 100 pointing at the first 10 elements of the array. In contrast, "new([]int)" returns a pointer to a newly allocated, zeroed slice structure, that is, a pointer to a "nil" slice value. These examples illustrate the difference between "new" and "make". Gieben Expires February 26, 2019 [Page 55] Internet-Draft Learning Go August 2018 var p *[]int = new([]int) <1> var v []int = make([]int, 100) <2> var p *[]int = new([]int) <3> *p = make([]int, 100, 100) v := make([]int, 100) <4> Allocates _1_ slice structure; rarely useful. "v" _2_ refers to a new array of 100 ints. At _3_ we make it unnecessarily complex, _4_ is more idiomatic. Remember that "make" applies only to maps, slices and channels and does not return a pointer. To obtain an explicit pointer allocate with "new". *new* allocates; *make* initializes. The above two paragraphs can be summarized as: o "new(T)" returns "*T" pointing to a zeroed "T" o "make(T)" returns an initialized "T" And of course "make" is only used for slices, maps and channels. 5.1.3. Constructors and composite literals Sometimes the zero value isn't good enough and an initializing constructor is necessary, as in this example taken from the package "os". func NewFile(fd int, name string) *File { if fd < 0 { return nil } f := new(File) f.fd = fd f.name = name f.dirinfo = nil f.nepipe = 0 return f } There's a lot of boiler plate in there. We can simplify it using a _composite literal_ , which is an expression that creates a new instance each time it is evaluated. Gieben Expires February 26, 2019 [Page 56] Internet-Draft Learning Go August 2018 func NewFile(fd int, name string) *File { if fd < 0 { return nil } f := File{fd, name, nil, 0} return &f <1> } It is OK to return the address of a local variable _1_ the storage associated with the variable survives after the function returns. In fact, taking the address of a composite literal allocates a fresh instance each time it is evaluated, so we can combine these last two lines. return &File{fd, name, nil, 0} The items (called fields) of a composite literal are laid out in order and must all be present. However, by labeling the elements explicitly as field:value pairs, the initializers can appear in any order, with the missing ones left as their respective zero values. Thus we could say return &File{fd: fd, name: name} As a limiting case, if a composite literal contains no fields at all, it creates a zero value for the type. The expressions "new(File)" and "&File{}" are equivalent. In fact the use of "new" is discouraged. Composite literals can also be created for arrays, slices, and maps, with the field labels being indices or map keys as appropriate. In these examples, the initializations work regardless of the values of "Enone", and "Einval", as long as they are distinct: ar := [...]string{Enone: "no error", Einval: "invalid argument"} sl := []string{Enone: "no error", Einval: "invalid argument"} ma := map[int]string {Enone: "no error", Einval: "invalid argument"} 5.2. Defining your own types Of course Go allows you to define new types, it does this with the "type" keyword: "type foo int" This creates a new type "foo" which acts like an "int". Creating more sophisticated types is done with the "struct" keyword. An example would be when we want record somebody's name ("string") and age ("int") in a single structure and make it a new type: Gieben Expires February 26, 2019 [Page 57] Internet-Draft Learning Go August 2018 package main import "fmt" type NameAge struct { name string // Both non exported fields. age int } func main() { a := new(NameAge) a.name = "Pete" a.age = 42 fmt.Printf("%v\n", a) } Apropos, the output of "fmt.Printf("%v\n", a)" is "&{Pete 42}" That is nice! Go knows how to print your structure. If you only want to print one, or a few, fields of the structure you'll need to use ".". For example to only print the name: fmt.Printf("%s", a.name) 5.2.1. More on structure fields As said each item in a structure is called a field. A struct with no fields: "struct {}". Or one with four fields: struct { x, y int A *[]int F func() } If you omit the name for a field, you create an anonymous field (((field, anonymous))), for instance: struct { T1 // Field name is T1. *T2 // Field name is T2. P.T3 // Field name is T3. x, y int // Field names are x and y. } Note that field names that start with a capital letter are exported, i.e. can be set or read from other packages. Field names that start Gieben Expires February 26, 2019 [Page 58] Internet-Draft Learning Go August 2018 with a lowercase are private to the current package. The same goes for functions defined in packages, see Section 4 for the details. 5.2.2. Methods If you create functions that work on your newly defined type, you can take two routes: 1. Create a function that takes the type as an argument. func doSomething(n1 *NameAge, n2 int) { /* */ } 1. Create a function that works on the type (see _receiver_ in Section 3): func (n1 *NameAge) doSomething(n2 int) { /* */ } This is a method call, which can be used as: var n *NameAge n.doSomething(2) Whether to use a function or method is entirely up to the programmer, but if you want to satisfy an interface (see the next chapter) you must use methods. If no such requirement exists it is a matter of taste whether to use functions or methods. But keep the following in mind, this is quoted from [go_spec]: If "x" is addressable and "&x"'s method set contains "m", "x.m()" is shorthand for "(&x).m()". In the above case this means that the following is _not_ an error: var n NameAge // Not a pointer n.doSomething(2) Here Go will search the method list for "n" of type "NameAge", come up empty and will then _also_ search the method list for the type "*NameAge" and will translate this call to "(&n).doSomething(2)". There is a subtle but major difference between the following type declarations. Also see the Section "Type Declarations" [go_spec]. Suppose we have: Gieben Expires February 26, 2019 [Page 59] Internet-Draft Learning Go August 2018 // A Mutex is a data type with two methods, Lock and Unlock. type Mutex struct { /* Mutex fields */ } func (m *Mutex) Lock() { /* Lock impl. */ } func (m *Mutex) Unlock() { /* Unlock impl. */ } We now create two types in two different manners: o "type NewMutex Mutex". o "type PrintableMutex struct{Mutex}". "NewMutex" is equal to "Mutex", but it _does not_ have _any_ of the methods of "Mutex". In other words its method set is empty. But "PrintableMutex" _has_ _inherited_ the method set from "Mutex". The Go term for this is _embedding_ . In the words of [go_spec]: The method set of "*PrintableMutex" contains the methods "Lock" and "Unlock" bound to its anonymous field "Mutex". 5.3. Conversions Sometimes you want to convert a type to another type. This is possible in Go, but there are some rules. For starters, converting from one value to another is done by operators (that look like functions: "byte()") and not all conversions are allowed. +-------+---------+---------+---------+---------+--------+----------+ | From | "b | "i | "r | "s | "f flo | "i int" | | | []byte" | []int" | []rune" | string" | at32" | | +-------+---------+---------+---------+---------+--------+----------+ | *To* | | | | | | | | "[]by | . | | | "[]byte | | | | te" | | | | (s)" | | | | "[]in | | . | | "[]int( | | | | t" | | | | s)" | | | | "[]ru | | | | "[]rune | | | | ne" | | | | (s)" | | | | "stri | "string | "string | "string | . | | | | ng" | (b)" | (i)" | (r)" | | | | | "floa | | | | | . | "float32 | | t32" | | | | | | (i)" | | "int" | | | | | "int(f | . | | | | | | | )" | | +-------+---------+---------+---------+---------+--------+----------+ Table 4: Valid conversions, float64 works the same as float32. o From a "string" to a slice of bytes or runes. Gieben Expires February 26, 2019 [Page 60] Internet-Draft Learning Go August 2018 mystring := "hello this is string" byteslice := []byte(mystring) Converts to a "byte" slice, each "byte" contains the integer value of the corresponding byte in the string. Note that as strings in Go are encoded in UTF-8 some characters in the string may end up in 1, 2, 3 or 4 bytes. runeslice := []rune(mystring) Converts to an "rune" slice, each "rune" contains a Unicode code point. Every character from the string corresponds to one rune. o From a slice of bytes or runes to a "string". b := []byte{'h','e','l','l','o'} // Composite literal. s := string(b) i := []rune{257,1024,65} r := string(i) For numeric values the following conversions are defined: o Convert to an integer with a specific (bit) length: "uint8(int)" o From floating point to an integer value: "int(float32)". This discards the fraction part from the floating point value. o And the other way around: "float32(int)". 5.3.1. User defined types and conversions How can you convert between the types you have defined yourself? We create two types here "Foo" and "Bar", where "Bar" is an alias for "Foo": type foo struct { int } // Anonymous struct field. type bar foo // bar is an alias for foo. Then we: var b bar = bar{1} // Declare `b` to be a `bar`. var f foo = b // Assign `b` to `f`. Which fails on the last line with: "cannot use b (type bar) as type foo in assignment" This can be fixed with a conversion: "var f foo = foo(b)" Note that converting structures that are not identical in their fields is more difficult. Also note that converting "b" to a plain Gieben Expires February 26, 2019 [Page 61] Internet-Draft Learning Go August 2018 "int" also fails; an integer is not the same as a structure containing an integer. 5.4. Exercises 5.4.1. Map function with interfaces 1. Use the answer from the earlier map exercise but now make it generic using interfaces. Make it at least work for ints and strings. 5.4.2. Answer 1. package main import "fmt" // Define the empty interface as a type. type e interface{} func mult2(f e) e { switch f.(type) { case int: return f.(int) * 2 case string: return f.(string) + f.(string) + f.(string) + f.(string) } return f } func Map(n []e, f func(e) e) []e { m := make([]e, len(n)) for k, v := range n { m[k] = f(v) } return m } func main() { m := []e{1, 2, 3, 4} s := []e{"a", "b", "c", "d"} mf := Map(m, mult2) sf := Map(s, mult2) fmt.Printf("%v\n", mf) fmt.Printf("%v\n", sf) } Gieben Expires February 26, 2019 [Page 62] Internet-Draft Learning Go August 2018 5.4.3. Pointers 1. Suppose we have defined the following structure: type Person struct { name string age int } What is the difference between the following two lines? var p1 Person p2 := new(Person) 2. What is the difference between the following two allocations? func Set(t *T) { x = t } and func Set(t T) { x= &t } 5.4.4. Answer 1. The expression, "var p1 Person" allocates a "Person"-_value_ to "p1". The type of "p1" is "Person". The second line: "p2 := new(Person)" allocates memory and assigns a _pointer_ to "p2". The type of "p2" is "*Person". 2. In the first function, "x" points to the same thing that "t" does, which is the same thing that the actual argument points to. So in the second function, we have an "extra" variable containing a copy of the interesting value. In the second function, "x" points to a new (heap-allocated) variable "t" which contains a copy of whatever the actual argument value is. 5.4.5. Linked List 1. Make use of the package "container/list" to create a (doubly) linked list. Push the values 1, 2 and 4 to the list and then print it. 2. Create your own linked list implementation. And perform the same actions as above. Gieben Expires February 26, 2019 [Page 63] Internet-Draft Learning Go August 2018 5.4.6. Answer 1. The following is the implementation of a program using doubly linked lists from "container/list". package main import ( "container/list" "fmt" ) func main() { l := list.New() l.PushBack(1) l.PushBack(2) l.PushBack(4) for e := l.Front(); e != nil; e = e.Next() { fmt.Printf("%v\n", e.Value) } } 1. The following is a program implementing a simple doubly linked list supporting "int" values. package main import ( "errors" <1> "fmt" ) type Value int <2> type Node struct { <3> Value prev, next *Node } type List struct { head, tail *Node } func (l *List) Front() *Node { <4> return l.head } Gieben Expires February 26, 2019 [Page 64] Internet-Draft Learning Go August 2018 func (n *Node) Next() *Node { return n.next } func (l *List) Push(v Value) *List { n := &Node{Value: v} <5> if l.head == nil { <6> l.head = n } else { l.tail.next = n <7> n.prev = l.tail <8> } l.tail = n <9> return l } var errEmpty = errors.New("List is empty") func (l *List) Pop() (v Value, err error) { if l.tail == nil { <10> err = errEmpty } else { v = l.tail.Value <11> l.tail = l.tail.prev <12> if l.tail == nil { l.head = nil <13> } } return v, err } func main() { l := new(List) l.Push(1) l.Push(2) l.Push(4) for n := l.Front(); n != nil; n = n.Next() { fmt.Printf("%v\n", n.Value) } fmt.Println() for v, err := l.Pop(); err == nil; v, err = l.Pop() { Gieben Expires February 26, 2019 [Page 65] Internet-Draft Learning Go August 2018 fmt.Printf("%v\n", v) } } Import <_1_> the packages we will need. At <_2_> we declare a type for the value our list will contain, this is not strictly neccesary. And at <_3_> we declare a type for the each node in our list. At <_4_> we define the "Front" method for our list. When pushing, create a new Node <_5_> with the provided value. If the list is empty <_6_>, put the new node at the head. Otherwise <_7_> put it at the tail and make sure <_8_> the new node points back to the previously existing one. At <_9_> we re-adjust tail to the newly inserted node. In the Pop _10_ method, we return an error if the list is empty. If it is not empty _11_ we save the last value. And then _12_ discard the last node from the list. Finally at _13_ we make sure the list is consistent if it becomes empty. 5.4.7. Cat 1. Write a program which mimics the Unix program "cat". 2. Make it support the "-n" flag, where each line is numbered. 3. The solution to the above question given in contains a bug. Can you spot and fix it? 5.4.8. Answer 1. The following is implemention of "cat" which also supports a -n flag to number each line. Gieben Expires February 26, 2019 [Page 66] Internet-Draft Learning Go August 2018 package main import ( "bufio" "flag" "fmt" "io" <1> "os" ) var numberFlag = flag.Bool("n", false, "number each line") // <<2>> func cat(r *bufio.Reader) { <3> i := 1 for { buf, e := r.ReadBytes('\n') <4> if e == io.EOF { <5> break } if *numberFlag { <6> fmt.Fprintf(os.Stdout, "%5d %s", i, buf) i++ } else { <7> fmt.Fprintf(os.Stdout, "%s", buf) } } return } func main() { flag.Parse() if flag.NArg() == 0 { cat(bufio.NewReader(os.Stdin)) } for i := 0; i < flag.NArg(); i++ { f, e := os.Open(flag.Arg(i)) if e != nil { fmt.Fprintf(os.Stderr, "%s: error reading from %s: %s\n", os.Args[0], flag.Arg(i), e.Error()) continue } cat(bufio.NewReader(f)) } } At _1_ we include all the packages we need. Here _2_ we define a new flag "n", which defaults to off. Note that we get the help (-h) for free. Start the function _3_ that actually reads the file's contents Gieben Expires February 26, 2019 [Page 67] Internet-Draft Learning Go August 2018 and displays it; Read one line at the time at _4_. And stop _5_ if we hit the end. If we should number each line, print the line number and then the line itself _6_. Otherwise _7_ we could just print the line. 1. The bug show itself when the last line of the input does not contain a newline. Or worse, when the input contains one line without a closing newline nothing is shown at all. A better solution is the following program. Gieben Expires February 26, 2019 [Page 68] Internet-Draft Learning Go August 2018 package main import ( "bufio" "flag" "fmt" "io" "os" ) var numberFlag = flag.Bool("n", false, "number each line") func cat(r *bufio.Reader) { i := 1 for { buf, e := r.ReadBytes('\n') if e == io.EOF && string(buf) == "" { break } if *numberFlag { fmt.Fprintf(os.Stdout, "%5d %s", i, buf) i++ } else { fmt.Fprintf(os.Stdout, "%s", buf) } } return } func main() { flag.Parse() if flag.NArg() == 0 { cat(bufio.NewReader(os.Stdin)) } for i := 0; i < flag.NArg(); i++ { f, e := os.Open(flag.Arg(i)) if e != nil { fmt.Fprintf(os.Stderr, "%s: error reading from %s: %s\n", os.Args[0], flag.Arg(i), e.Error()) continue } cat(bufio.NewReader(f)) } } Gieben Expires February 26, 2019 [Page 69] Internet-Draft Learning Go August 2018 5.4.9. Method calls 1. Suppose we have the following program. Note the package "container/vector" was once part of Go, but was removed when the "append" built-in was introduced. However, for this question this isn't important. The package implemented a stack-like structure, with push and pop methods. package main import "container/vector" func main() { k1 := vector.IntVector{} k2 := &vector.IntVector{} k3 := new(vector.IntVector) k1.Push(2) k2.Push(3) k3.Push(4) } What are the types of "k1", "k2" and "k3"? 2. Now, this program compiles and runs OK. All the "Push" operations work even though the variables are of a different type. The documentation for "Push" says: 2. So the receiver has to be of type "*IntVector", why does the code above (the Push statements) work correctly then? 5.4.10. Answer 1. The type of "k1" is "vector.IntVector". Why? We use a composite literal (the "{}"), so we get a value of that type back. The variable "k2" is of "*vector.IntVector", because we take the address ("&") of the composite literal. And finally "k3" has also the type "*vector.IntVector", because "new" returns a pointer to the type. 2. The answer is given in [go_spec] in the section "Calls", where among other things it says: A method call "x.m()" is valid if the method set of (the type of) "x" contains "m" and the argument list can be assigned to the parameter list of "m". If "x" is addressable and "&x"'s method set contains "m", "x.m()" is shorthand for "(&x).m()". Gieben Expires February 26, 2019 [Page 70] Internet-Draft Learning Go August 2018 In other words because "k1" is addressable and "*vector.IntVector" _does_ have the "Push" method, the call "k1.Push(2)" is translated by Go into "(&k1).Push(2)" which makes the type system happy again (and you too -- now you know this). 6. Interfaces I have this phobia about having my body penetrated surgically. You know what I mean? ------------------------------------------------------------ eXistenZ -- Ted Pikul In Go, the word _interface_ is overloaded to mean several different things. Every type has an interface, which is the _set of methods defined_ for that type. This bit of code defines a struct type "S" with one field, and defines two methods for "S". type S struct { i int } func (p *S) Get() int { return p.i } func (p *S) Put(v int) { p.i = v } Defining a struct and methods on it. You can also define an interface type, which is simply a set of methods. This defines an interface "I" with two methods: type I interface { Get() int Put(int) } "S" is a valid _implementation_ for interface "I", because it defines the two methods which "I" requires. Note that this is true even though there is no explicit declaration that "S" implements "I". A Go program can use this fact via yet another meaning of interface, which is an interface value: func f(p I) { <1> fmt.Println(p.Get()) <2> p.Put(1) <3> } At _1_ we declare a function that takes an interface type as the argument. Because "p" implements "I", it _must_ have the "Get()" method, which we call at _2_. And the same holds true for the "Put()" Gieben Expires February 26, 2019 [Page 71] Internet-Draft Learning Go August 2018 method at _3_. Because "S" implements "I", we can call the function "f" passing in a pointer to a value of type "S": "var s S; f(&s)" The reason we need to take the address of "s", rather than a value of type "S", is because we defined the methods on "s" to operate on pointers, see the definition in the code above. This is not a requirement -- we could have defined the methods to take values -- but then the "Put" method would not work as expected. The fact that you do not need to declare whether or not a type implements an interface means that Go implements a form of duck typing [duck_typing]. This is not pure duck typing, because when possible the Go compiler will statically check whether the type implements the interface. However, Go does have a purely dynamic aspect, in that you can convert from one interface type to another. In the general case, that conversion is checked at run time. If the conversion is invalid -- if the type of the value stored in the existing interface value does not satisfy the interface to which it is being converted -- the program will fail with a run time error. Interfaces in Go are similar to ideas in several other programming languages: pure abstract virtual base classes in C++, typeclasses in Haskell or duck typing in Python. However there is no other language which combines interface values, static type checking, dynamic run time conversion, and no requirement for explicitly declaring that a type satisfies an interface. The result in Go is powerful, flexible, efficient, and easy to write. 6.1. Which is what? Let's define another type "R" that also implements the interface "I": type R struct { i int } func (p *R) Get() int { return p.i } func (p *R) Put(v int) { p.i = v } The function "f" can now accept variables of type "R" and "S". Suppose you need to know the actual type in the function "f". In Go you can figure that out by using a type switch. func f(p I) { switch t := p.(type) { <1> case *S: <2> case *R: <2> default: <3> } } Gieben Expires February 26, 2019 [Page 72] Internet-Draft Learning Go August 2018 At _1_ we use the type switch, note that the ".(type)" syntax is _only_ valid within a "switch" statement. We store the value in the variable "t". The subsequent cases _2_ each check for a different _actual_ type. And we can even have a "default" _3_ clause. It is worth pointing out that both "case R" and "case s" aren't possible, because "p" needs to be a pointer in order to satisfy "i". A type switch isn't the only way to discover the type at _run-time_. if t, ok := something.(I); ok { <1> // ... } You can also use a "comma, ok" form _1_ to see if an interface type implements a specific interface. If "ok" is true, "t" will hold the type of "something". When you are sure a variable implements an interface you can use: "t := something.(I)" . 6.2. Empty interface Since every type satisfies the empty interface: "interface{}" we can create a generic function which has an empty interface as its argument: func g(something interface{}) int { return something.(I).Get() } The "return something.(I).Get()" is the tricky bit in this function. The value "something" has type "interface{}", meaning no guarantee of any methods at all: it could contain any type. The ".(I)" is a type assertion which converts "something" to an interface of type "I". If we have that type we can invoke the "Get()" function. So if we create a new variable of the type "*S", we can just call "g()", because "*S" also implements the empty interface. s = new(S) fmt.Println(g(s)); The call to "g" will work fine and will print 0. If we however invoke "g()" with a value that does not implement "I" we have a problem: var i int fmt.Println(g(i)) This compiles, but when we run this we get slammed with: "panic: interface conversion: int is not main.I: missing method Get". Gieben Expires February 26, 2019 [Page 73] Internet-Draft Learning Go August 2018 Which is completely true, the built-in type "int" does not have a "Get()" method. 6.3. Methods Methods are functions that have a receiver (see Section 3). You can define methods on any type (except on non-local types, this includes built-in types: the type "int" can not have methods). You can however make a new integer type with its own methods. For example: type Foo int func (self Foo) Emit() { fmt.Printf("%v", self) } type Emitter interface { Emit() } Doing this on non-local (types defined in other packages) types yields an error "cannot define new methods on non-local type int". 6.4. Methods on interface types An interface defines a set of methods. A method contains the actual code. In other words, an interface is the definition and the methods are the implementation. So a receiver can not be an interface type, doing so results in a "invalid receiver type ..." compiler error. The authoritative word from the language spec [go_spec]: The receiver type must be of the form "T" or "*T" where "T" is a type name. "T" is called the receiver base type or just base type. The base type must not be a pointer or interface type and must be declared in the same package as the method. Creating a pointer to an interface value is a useless action in Go. It is in fact illegal to create a pointer to an interface value. The release notes for an earlier Go release that made them illegal leave no room for doubt: The language change is that uses of pointers to interface values no longer automatically de-reference the pointer. A pointer to an interface value is more often a beginner's bug than correct code. Gieben Expires February 26, 2019 [Page 74] Internet-Draft Learning Go August 2018 6.5. Interface names By convention, one-method interfaces are named by the method name plus the _-er_ suffix: Read_er_, Writ_er_, Formatt_er_ etc. There are a number of such names and it's productive to honor them and the function names they capture. "Read", "Write", "Close", "Flush", "String" and so on have canonical signatures and meanings. To avoid confusion, don't give your method one of those names unless it has the same signature and meaning. Conversely, if your type implements a method with the same meaning as a method on a well-known type, give it the same name and signature; call your string-converter method "String" not "ToString". 6.6. A sorting example Recall the Bubblesort exercise, where we sorted an array of integers: func bubblesort(n []int) { for i := 0; i < len(n)-1; i++ { for j := i + 1; j < len(n); j++ { if n[j] < n[i] { n[i], n[j] = n[j], n[i] } } } } A version that sorts strings is identical except for the signature of the function: "func bubblesortString(n []string) { /* ... */ }" . Using this approach would lead to two functions, one for each type. By using interfaces we can make this more generic. Let's create a new function that will sort both strings and integers, something along the lines of this non-working example: func sort(i []interface{}) { <1> switch i.(type) { <2> case string: <3> // ... case int: // ... } return /* ... */ <4> } Our function will receive a slice of empty interfaces at _1_. We then _2_ use a type switch to find out what the actual type of the input Gieben Expires February 26, 2019 [Page 75] Internet-Draft Learning Go August 2018 is. And then _3_ then sort accordingly. And, when done, return _4_ the sorted slice. But when we call this function with "sort([]int{1, 4, 5})", it fails with: "cannot use i (type []int) as type []interface { } in function argument" This is because Go can not easily convert to a _slice_ of interfaces. Just converting to an interface is easy, but to a slice is much more costly. The full mailing list discussion on this subject can be found at [go_nuts_interfaces]. To keep a long story short: Go does not (implicitly) convert slices for you. So what is the Go way of creating such a "generic" function? Instead of doing the type inference ourselves with a type switch, we let Go do it implicitly: The following steps are required: o Define an interface type (called "Sorter" here) with a number of methods needed for sorting. We will at least need a function to get the length of the slice, a function to compare two values and a swap function. type Sorter interface { Len() int // len() as a method. Less(i, j int) bool // p[j] < p[i] as a method. Swap(i, j int) // p[i], p[j] = p[j], p[i] as a method. } o Define new types for the slices we want to sort. Note that we declare slice types: type Xi []int type Xs []string o Implementation of the methods of the "Sorter" interface. For integers: func (p Xi) Len() int {return len(p)} func (p Xi) Less(i int, j int) bool {return p[j] < p[i]} func (p Xi) Swap(i int, j int) {p[i], p[j] = p[j], p[i]} And for strings: func (p Xs) Len() int {return len(p)} func (p Xs) Less(i int, j int) bool {return p[j] < p[i]} func (p Xs) Swap(i int, j int) {p[i], p[j] = p[j], p[i]} o Write a _generic_ Sort function that works on the "Sorter" interface. Gieben Expires February 26, 2019 [Page 76] Internet-Draft Learning Go August 2018 func Sort(x Sorter) { <1> for i := 0; i < x.Len() - 1; i++ { <2> for j := i + 1; j < x.Len(); j++ { if x.Less(i, j) { x.Swap(i, j) } } } } At _1_ "x" is now of the "Sorter" type and using the defined methods for this interface we implement Bubblesort at _2_. Now we can use our _generic_ "Sort" function as follows: ints := Xi{44, 67, 3, 17, 89, 10, 73, 9, 14, 8} strings := Xs{"nut", "ape", "elephant", "zoo", "go"} Sort(ints) fmt.Printf("%v\n", ints) Sort(strings) fmt.Printf("%v\n", strings) 6.7. Listing interfaces in interfaces Take a look at the following example of an interface definition, this one is from the package "container/heap": type Interface interface { sort.Interface Push(x interface{}) Pop() interface{} } Here another interface is listed inside the definition of "heap.Interface", this may look odd, but is perfectly valid, remember that on the surface an interface is nothing more than a listing of methods. "sort.Interface" is also such a listing, so it is perfectly legal to include it in the interface. 6.8. Introspection and reflection In the following example we want to look at the "tag" (here named "namestr") defined in the type definition of "Person". To do this we need the "reflect" package (there is no other way in Go). Keep in mind that looking at a tag means going back to the _type_ definition. So we use the "reflect" package to figure out the type of the variable and _then_ access the tag. Gieben Expires February 26, 2019 [Page 77] Internet-Draft Learning Go August 2018 type Person struct { name string "namestr" age int } func ShowTag(i interface{}) { <1> switch t := reflect.TypeOf(i); t.Kind() { case reflect.Ptr: <2> tag := t.Elem().Field(0).Tag // <<3>> <<4>> <<5>> Introspection using reflection. We are calling "ShowTag" at _1_ with a "*Person", so at _2_ we're expecting a "reflect.Ptr". We are dealing with a "Type" _3_ and according to the documentation : Elem returns a type's element type. It panics if the type's Kind is not Array, Chan, Map, Ptr, or Slice. So on "t" we use "Elem()" to get the value the pointer points to. We have now dereferenced the pointer and are "inside" our structure. We then _4_ use "Field(0)" to access the zeroth field. The struct "StructField" has a "Tag" member which returns the tag- name as a string. So on the "0^{th}" field we can unleash ".Tag" _5_ to access this name: "Field(0).Tag". This gives us "namestr". To make the difference between types and values more clear, take a look at the following code: func show(i interface{}) { switch t := i.(type) { case *Person: t := reflect.TypeOf(i) <1> v := reflect.ValueOf(i) <2> tag := t.Elem().Field(0).Tag <3> name := v.Elem().Field(0).String() <4> } } Reflection and the type and value. At _1_ we create "t" the type data of "i", and "v" gets the actual values at _2_. Here at _3_ we want to get to the "tag". So we need "Elem()" to redirect the pointer, access the first field and get the tag. Note that we operate on "t" a "reflect.Type". Now _4_ we want to get access to the _value_ of one of the members and we employ Gieben Expires February 26, 2019 [Page 78] Internet-Draft Learning Go August 2018 "Elem()" on "v" to do the redirection. we have "arrived" at the structure. Then we go to the first field "Field(0)" and invoke the "String()" method on it. fig/reflection.png Peeling away the layers using reflection. Peeling away the layers using reflection. Going from a *Person via Elem using the methods described in go doc reflect to get the actual string contained within.") Setting a value works similarly as getting a value, but only works on _exported_ members. Again some code: type Person struct { name string age int } func Set(i interface{}) { switch i.(type) { case *Person: r := reflect.ValueOf(i) r.Elem(0).Field(0).SetString("Albert Einstein") } } Reflect with private member. type Person struct { Name string age int } func Set(i interface{}) { switch i.(type) { case *Person: r := reflect.ValueOf(i) r.Elem().Field(0).SetString("Albert Einstein") } } Reflect with public member. The first program compiles and runs, but when you run it, you are greeted with a stack trace and a _run time_ error: "panic: reflect.Value.SetString using value obtained using unexported field". Gieben Expires February 26, 2019 [Page 79] Internet-Draft Learning Go August 2018 The second program works OK and sets the member "Name" to "Albert Einstein". Of course this only works when you call "Set()" with a pointer argument. 6.9. Exercises ### Interfaces and max() In the maximum exercise we created a max function that works on a slice of integers. The question now is to create a program that shows the maximum number and that works for both integers and floats. Try to make your program as generic as possible, although that is quite difficult in this case. 6.9.1. Answer The following program calculates a maximum. It is as generic as you can get with Go. Gieben Expires February 26, 2019 [Page 80] Internet-Draft Learning Go August 2018 package main import "fmt" func Less(l, r interface{}) bool { <1> switch l.(type) { case int: if _, ok := r.(int); ok { return l.(int) < r.(int) <2> } case float32: if _, ok := r.(float32); ok { return l.(float32) < r.(float32) <3> } } return false } func main() { var a, b, c int = 5, 15, 0 var x, y, z float32 = 5.4, 29.3, 0.0 if c = a; Less(a, b) { <4> c = b } if z = x; Less(x, y) { <4> z = y } fmt.Println(c, z) } We could have chosen to make the return type of this _1_ function an "interface{}", but that would mean that a caller would always have to do a type assertion to extract the actual type from the interface. At _2_ we compare the parameters. All parameters are confirmed to be integers, so this is legit. And at _3_ we do the some for floats. At _4_ we get the maximum value for "a", "b" and "x" and "y". 6.9.2. Pointers and reflection One of the last paragraphs in section Section 6.8 has the following words: The code on the right works OK and sets the member "Name" to "Albert Einstein". Of course this only works when you call "Set()" with a pointer argument. Why is this the case? Gieben Expires February 26, 2019 [Page 81] Internet-Draft Learning Go August 2018 6.9.3. Answer When called with a non-pointer argument the variable is a copy (call- by-value). So you are doing the reflection voodoo on a copy. And thus you are _not_ changing the original value, but only this copy. 7. Concurrency * Parallelism is about performance. * Concurrency is about program design. **** Google I/O 2010 -- Rob Pike In this chapter we will show off Go's ability for concurrent programming using channels and goroutines. Goroutines are the central entity in Go's ability for concurrency. But what _is_ a goroutine, from [effective_go]: They're called goroutines because the existing terms -- threads, coroutines, processes, and so on -- convey inaccurate connotations. A goroutine has a simple model: _it is a function executing in parallel with other goroutines in the same address space_. It is lightweight, costing little more than the allocation of stack space. And the stacks start small, so they are cheap, and grow by allocating (and freeing) heap storage as required. A goroutine is a normal function, except that you start it with the keyword "go". ready("Tea", 2) // Normal function call. go ready("Tea", 2) // ... as goroutine. func ready(w string, sec int) { time.Sleep(time.Duration(sec) * time.Second) fmt.Println(w, "is ready!") } func main() { go ready("Tea", 2) //<1> go ready("Coffee", 1) //<1> fmt.Println("I'm waiting") time.Sleep(5 * time.Second) //<2> Figure: Go routines in action. Gieben Expires February 26, 2019 [Page 82] Internet-Draft Learning Go August 2018 The following idea for a program was taken from [go_course_day3]. We run a function as two goroutines, the goroutines wait for an amount of time and then print something to the screen. At _1_ we start the goroutines. The "main" function waits long enough at _2_, so that both goroutines will have printed their text. Right now we wait for 5 seconds, but in fact we have no idea how long we should wait until all goroutines have exited. This outputs: I'm waiting // Right away Coffee is ready! // After 1 second Tea is ready! // After 2 seconds If we did not wait for the goroutines (i.e. remove the last line at _2_) the program would be terminated immediately and any running goroutines would _die with it_. To fix this we need some kind of mechanism which allows us to communicate with the goroutines. This mechanism is available to us in the form of channels . A channel can be compared to a two-way pipe in Unix shells: you can send to and receive values from it. Those values can only be of a specific type: the type of the channel. If we define a channel, we must also define the type of the values we can send on the channel. Note that we must use "make" to create a channel: ci := make(chan int) cs := make(chan string) cf := make(chan interface{}) Makes "ci" a channel on which we can send and receive integers, makes "cs" a channel for strings and "cf" a channel for types that satisfy the empty interface. Sending on a channel and receiving from it, is done with the same operator: "<-". Depending on the operands it figures out what to do: ci <- 1 // *Send* the integer 1 to the channel ci. <-ci // *Receive* an integer from the channel ci. i := <-ci // *Receive* from the channel ci and store it in i. Let's put this to use. Gieben Expires February 26, 2019 [Page 83] Internet-Draft Learning Go August 2018 var c chan int <1> func ready(w string, sec int) { time.Sleep(time.Duration(sec) * time.Second) fmt.Println(w, "is ready!") c <- 1 <2> } func main() { c = make(chan int) <3> go ready("Tea", 2) <4> go ready("Coffee", 1) <4> fmt.Println("I'm waiting, but not too long") <-c <5> <-c <5> } At _1_ we declare "c" to be a variable that is a channel of ints. That is: this channel can move integers. Note that this variable is global so that the goroutines have access to it. At _2_ in the "ready" function we send the integer 1 on the channel. In our "main" function we initialize "c" at _3_ and start our goroutines _4_. At _5_ we Wait until we receive a value from the channel, the value we receive is discarded. We have started two goroutines, so we expect two values to receive. There is still some remaining ugliness; we have to read twice from the channel _5_). This is OK in this case, but what if we don't know how many goroutines we started? This is where another Go built-in comes in: "select" (((keywords, select))). With "select" you can (among other things) listen for incoming data on a channel. Using "select" in our program does not really make it shorter, because we run too few go-routines. We remove last lines and replace them with the following: L: for { select { case <-c: i++ if i > 1 { break L } } } We will now wait as long as it takes. Only when we have received more than one reply on the channel "c" will we exit the loop "L". Gieben Expires February 26, 2019 [Page 84] Internet-Draft Learning Go August 2018 7.1. Make it run in parallel While our goroutines were running concurrently, they were not running in parallel. When you do not tell Go anything there can only be one goroutine running at a time. With "runtime.GOMAXPROCS(n)" you can set the number of goroutines that can run in parallel. From the documentation: GOMAXPROCS sets the maximum number of CPUs that can be executing simultaneously and returns the previous setting. If n < 1, it does not change the current setting. _This call will go away when the scheduler improves._ If you do not want to change any source code you can also set an environment variable "GOMAXPROCS" to the desired value. Note that the above discussion relates to older versions of Go. From version 1.5 and above, "GOMAXPROCS" defaults to the number of CPU cores[go_1_5_release_notes]. 7.2. More on channels When you create a channel in Go with "ch := make(chan bool)", an unbuffered channel for bools is created. What does this mean for your program? For one, if you read ("value := <-ch") it will block until there is data to receive. Secondly anything sending ("ch <- true") will block until there is somebody to read it. Unbuffered channels make a perfect tool for synchronizing multiple goroutines. But Go allows you to specify the buffer size of a channel, which is quite simply how many elements a channel can hold. "ch := make(chan bool, 4)", creates a buffered channel of bools that can hold 4 elements. The first 4 elements in this channel are written without any blocking. When you write the 5^(th) In conclusion, the following is true in Go: \textsf{ch := make(chan type, value)} \left\{ \begin{array}{ll} value == 0 & \rightarrow \textsf{unbuffered} \\ value > 0 & \rightarrow \textsf{buffer }{} value{} \textsf{ elements} \end{array} \right. Gieben Expires February 26, 2019 [Page 85] Internet-Draft Learning Go August 2018 \textsf{ch := make(chan type, value)} \left\{ \begin{array}{ll} value == 0 & \rightarrow \textsf{unbuffered} \\ value > 0 & \rightarrow \textsf{buffer }{} value{} \textsf{ elements} \end{array} \right. When a channel is closed the reading side needs to know this. The following code will check if a channel is closed. x, ok = <-ch Where "ok" is set to "true" the channel is not closed _and_ we've read something. Otherwise "ok" is set to "false". In that case the channel was closed and the value received is a zero value of the channel's type. 7.3. Exercises 7.3.1. Channels 1. Modify the program you created in exercise Section 2.9.1 to use channels, in other words, the function called in the body should now be a goroutine and communication should happen via channels. You should not worry yourself on how the goroutine terminates. 2. There are a few annoying issues left if you resolve question 1 above. One of the problems is that the goroutine isn't neatly cleaned up when "main.main()" exits. And worse, due to a race condition between the exit of "main.main()" and "main.shower()" not all numbers are printed. It should print up until 9, but sometimes it prints only to 8. Adding a second quit-channel you can remedy both issues. Do this. 7.3.2. Answer 1. A possible program is: Gieben Expires February 26, 2019 [Page 86] Internet-Draft Learning Go August 2018 package main import "fmt" func main() { ch := make(chan int) go shower(ch) for i := 0; i < 10; i++ { ch <- i } } func shower(c chan int) { for { j := <-c fmt.Printf("%d\n", j) } } We start in the usual way, then at line 6 we create a new channel of ints. In the next line we fire off the function "shower" with the "ch" variable as it argument, so that we may communicate with it. Next we start our for-loop (lines 8-10) and in the loop we send (with "<-") our number to the function (now a goroutine) "shower". In the function "shower" we wait (as this blocks) until we receive a number (line 15). Any received number is printed (line 16) and then continue the endless loop started on line 14. 1. An answer is Gieben Expires February 26, 2019 [Page 87] Internet-Draft Learning Go August 2018 package main import "fmt" func main() { ch := make(chan int) quit := make(chan bool) go shower(ch, quit) for i := 0; i < 10; i++ { ch <- i } quit <- false // or true, does not matter } func shower(c chan int, quit chan bool) { for { select { case j := <-c: fmt.Printf("%d\n", j) case <-quit: break } } } On line 20 we read from the quit channel and we discard the value we read. We could have used "q := <-quit", but then we would have used the variable only once --- which is illegal in Go. Another trick you might have pulled out of your hat may be: "_ = <-quit". This is valid in Go, but idomatic Go is the one given on line 20. 7.3.3. Fibonacci II This is the same exercise as an earlier one Section 3.7.7 in exercise. For completeness the complete question: The Fibonacci sequence starts as follows: "1, 1, 2, 3, 5, 8, 13, \ldots" Or in mathematical terms: "x_1 = 1; x_2 = 1; x_n = x_{n-1} + > x_{n-2}\quad\forall n > 2". Write a function that takes an "int" value and gives that many terms of the Fibonacci sequence. _But_ now the twist: You must use channels. Gieben Expires February 26, 2019 [Page 88] Internet-Draft Learning Go August 2018 7.3.4. Answer The following program calculates the Fibonacci numbers using channels. package main import "fmt" func dup3(in <-chan int) (<-chan int, <-chan int, <-chan int) { a, b, c := make(chan int, 2), make(chan int, 2), make(chan int, 2) go func() { for { x := <-in a <- x b <- x c <- x } }() return a, b, c } func fib() <-chan int { x := make(chan int, 2) a, b, out := dup3(x) go func() { x <- 0 x <- 1 <-a for { x <- <-a+<-b } }() return out } func main() { x := fib() for i := 0; i < 10; i++ { fmt.Println(<-x) } } // See sdh33b.blogspot.com/2009/12/fibonacci-in-go.html Gieben Expires February 26, 2019 [Page 89] Internet-Draft Learning Go August 2018 8. Communication Good communication is as stimulating as black coffee, and just as hard to sleep after. ------------------------------------------------------------ -- Anne Morrow Lindbergh In this chapter we are going to look at the building blocks in Go for communicating with the outside world. We will look at files, directories, networking and executing other programs. Central to Go's I/O are the interfaces "io.Reader" and "io.Writer". The "io.Reader" interface specifies one method "Read(p []byte) (n int, err err)". Reading from (and writing to) files is easy in Go. This program only uses the "os" package to read data from the file "/etc/passwd". package main import ( "log" "os" ) func main() { buf := make([]byte, 1024) f, e := os.Open("/etc/passwd") <1> if e != nil { log.Fatalf(e) } defer f.Close() <2> for { n, e := f.Read(buf) <3> if e != nil { log.Fatalf(e) <4> } if n == 0 { <5> break } os.Stdout.Write(buf[:n]) <6> } } We open the file at _1_ with "os.Open" that returns a "*os.File" "*os.File" implements "io.Reader" and "io.Writer" interface. After the "Open" we directly put the "f.Close()" which we defer until the Gieben Expires February 26, 2019 [Page 90] Internet-Draft Learning Go August 2018 function return. At _3_ we call "Read" on "f" and read up to 1024 bytes at the time. If anything fails we bail out at _4_. If the number of bytes read is 0 we've read the end of the file _5_. And at _6_ we output the buffer to standard output. If you want to use buffered I/O there is the "bufio" package: package main import ( "bufio" "log" "os" ) func main() { buf := make([]byte, 1024) f, e := os.Open("/etc/passwd") <1> if e != nil { log.Fatalf(e) } defer f.Close() r := bufio.NewReader(f) <2> w := bufio.NewWriter(os.Stdout) defer w.Flush() <3> for { n, e := r.Read(buf) <4> if e != nil { log.Fatalf(e) } if n == 0 { break } w.Write(buf[0:n]) <5> } } Again, we open _1_ the file. Then at _2_ we Turn "f" into a buffered "Reader". "NewReader" expects an "io.Reader", so you this will work. Then at _4_ we read and at _5_ we write. We also call "Flush()" at _3_ to flush all output. This entire program could be optimized further by using "io.Copy". 8.1. io.Reader As mentioned above the "io.Reader" is an important interface in the language Go. A lot (if not all) functions that need to read from something take an "io.Reader" as input. To fulfill the interface a Gieben Expires February 26, 2019 [Page 91] Internet-Draft Learning Go August 2018 type needs to implement that one method. The writing side "io.Writer", has the "Write" method. If you think of a new type in your program or package and you make it fulfill the "io.Reader" or "io.Writer" interface, _the whole standard Go library can be used_ on that type! 8.2. Some examples The previous program reads a file in its entirety, but a more common scenario is that you want to read a file on a line-by-line basis. The following snippet shows a way to do just that (we're discarding the error returned from "os.Open" here to keep the examples smaller -- don't ever do this in real life code). f, _ := os.Open("/etc/passwd"); defer f.Close() r := bufio.NewReader(f) <1> s, ok := r.ReadString('\n') <2> At _1_ make "f" a "bufio" to have access to the "ReadString" method. Then at _2_ we read a line from the input, "s" now holds a string which we can manipulate with, for instance, the "strings" package. A more robust method (but slightly more complicated) is "ReadLine", see the documentation of the "bufio" package. A common scenario in shell scripting is that you want to check if a directory exists and if not, create one. if [ ! -e name ]; then if f, e := os.Stat("name"); e != nil { mkdir name os.Mkdir("name", 0755) else } else { # error // error fi } The similarity between these two examples (and with other scripting languages) have prompted comments that Go has a "script"-like feel to it, i.e. programming in Go can be compared to programming in an interpreted language (Python, Ruby, Perl or PHP). 8.3. Command line arguments Arguments from the command line are available inside your program via the string slice "os.Args", provided you have imported the package "os". The "flag" package has a more sophisticated interface, and also provides a way to parse flags. Take this example from a DNS query tool: Gieben Expires February 26, 2019 [Page 92] Internet-Draft Learning Go August 2018 dnssec := flag.Bool("dnssec", false, "Request DNSSEC records") <1> port := flag.String("port", "53", "Set the query port") <2> flag.Usage = func() { <3> fmt.Fprintf(os.Stderr, "Usage: %s [OPTIONS] [name ...]\n", os.Args[0]) flag.PrintDefaults() <4> } flag.Parse() <4> At _1_ we define a "bool" flag "-dnssec". Note that this function returns a _pointer_ to the value, the "dnssec" is now a pointer to a "bool". At _2_ we define an "strings" flag. Then at _3_ we _redefine_ the "Usage" variable of the flag package so we can add some extra text. The "PrintDefaults" at _4_ will output the default help for the flags that are defined. Note even without redefining a "flag.Usage" the flag "-h" is supported and will just output the help text for each of the flags. Finally at _4_ we call "Parse" that parses the command line and fills the variables. After the flags have been parsed you can used them: "if *dnssec { ... }" 8.4. Executing commands The "os/exec" package has functions to run external commands, and is the premier way to execute commands from within a Go program. It works by defining a "*exec.Cmd" structure for which it defines a number of methods. Let's execute "ls -l": import "os/exec" cmd := exec.Command("/bin/ls", "-l") err := cmd.Run() The above example just runs "ls -l" without doing anything with the returned data, capturing the standard output from a command is done as follows: cmd := exec.Command("/bin/ls", "-l") buf, err := cmd.Output() And "buf" is byte slice, that you can further use in your program. 8.5. Networking All network related types and functions can be found in the package "net". One of the most important functions in there is "Dial". When you "Dial" into a remote system the function returns a "Conn" interface type, which can be used to send and receive information. Gieben Expires February 26, 2019 [Page 93] Internet-Draft Learning Go August 2018 The function "Dial" neatly abstracts away the network family and transport. So IPv4 or IPv6, TCP or UDP can all share a common interface. Dialing a remote system (port 80) over TCP, then UDP and lastly TCP over IPv6 looks like this: conn, e := Dial("tcp", "192.0.32.10:80") conn, e := Dial("udp", "192.0.32.10:80") conn, e := Dial("tcp", "[2620:0:2d0:200::10]:80") If there were no errors (returned in "e"), you can use "conn" to read and write. And "conn" implements the "io.Reader" and "io.Writer" interface. But these are the low level nooks and crannies, you will almost always use higher level packages, such as the "http" package. For instance a simple Get for http: package main import ( "fmt" "http" "io/ioutil" ) func main() { r, err := http.Get("http://www.google.com/robots.txt") if err != nil { fmt.Printf("%s\n", err.String()) return } b, err := ioutil.ReadAll(r.Body) r.Body.Close() if err == nil { fmt.Printf("%s", string(b)) } } 8.6. Exercises 8.6.1. Finger daemon Write a finger daemon that works with the finger(1) command. From the Debian [9] package description: Gieben Expires February 26, 2019 [Page 94] Internet-Draft Learning Go August 2018 Fingerd is a simple daemon based on RFC 1196 [RFC1196] that provides an interface to the "finger" program at most network sites. The program is supposed to return a friendly, human- oriented status report on either the system at the moment or a particular person in depth. Stick to the basics and only support a username argument. If the user has a ".plan" file show the contents of that file. So your program needs to be able to figure out: o Does the user exist? o If the user exists, show the contents of the ".plan" file. 8.6.2. Answer This solution is from Fabian Becker. package main import ( "bufio" "errors" "flag" "io/ioutil" "net" "os/user" ) func main() { flag.Parse() ln, err := net.Listen("tcp", ":79") if err != nil { panic(err) } for { conn, err := ln.Accept() if err != nil { continue } go handleConnection(conn) } } func handleConnection(conn net.Conn) { defer conn.Close() reader := bufio.NewReader(conn) usr, _, _ := reader.ReadLine() Gieben Expires February 26, 2019 [Page 95] Internet-Draft Learning Go August 2018 if info, err := getUserInfo(string(usr)); err != nil { conn.Write([]byte(err.Error())) } else { conn.Write(info) } } func getUserInfo(usr string) ([]byte, error) { u, e := user.Lookup(usr) if e != nil { return nil, e } data, err := ioutil.ReadFile(u.HomeDir + ".plan") if err != nil { return data, errors.New("User doesn't have a .plan file!\n") } return data, nil } 8.6.3. Echo server Write a simple echo server. Make it listen to TCP port number 8053 on localhost. It should be able to read a line (up to the newline), echo back that line and then close the connection. Make the server concurrent so that every request is taken care of in a separate goroutine. 8.6.4. Answer A simple echo server might be: Gieben Expires February 26, 2019 [Page 96] Internet-Draft Learning Go August 2018 package main import ( "bufio" "fmt" "net" ) func main() { l, err := net.Listen("tcp", "127.0.0.1:8053") if err != nil { fmt.Printf("Failure to listen: %s\n", err.Error()) } for { if c, err := l.Accept(); err == nil { Echo(c) } } } func Echo(c net.Conn) { defer c.Close() line, err := bufio.NewReader(c).ReadString('\n') if err != nil { fmt.Printf("Failure to read: %s\n", err.Error()) return } _, err = c.Write([]byte(line)) if err != nil { fmt.Printf("Failure to write: %s\n", err.Error()) return } } When started you should see the following: % nc 127.0.0.1 8053 Go is *awesome* Go is *awesome* To make the connection handling concurrent we _only need to change one line_ in our echo server, the line: if c, err := l.Accept(); err == nil { Echo(c) } becomes: if c, err := l.Accept(); err == nil { go Echo(c) } Gieben Expires February 26, 2019 [Page 97] Internet-Draft Learning Go August 2018 8.6.5. Word and Letter Count Write a small program that reads text from standard input and performs the following actions: o Count the number of characters (including spaces). o Count the number of words. o Count the numbers of lines In other words implement wc(1) (check you local manual page), however you only have to read from standard input. 8.6.6. Answer The following program is an implementation of wc(1). package main import ( "bufio" "fmt" "os" "strings" ) func main() { var chars, words, lines int r := bufio.NewReader(os.Stdin) <1> for { switch s, ok := r.ReadString('\n'); true { <2> case ok != nil: <3> fmt.Printf("%d %d %d\n", chars, words, lines) return default: <4> chars += len(s) words += len(strings.Fields(s)) lines++ } } } At _1_ we create a new reader that reads from standard input, we then read from the input at _2_. And at _3_ we check the value of "ok" and if we received an error, we assume it was because of a EOF, So we print the current values;. Otherwise _4_ we count the charaters, words and increment the number lines. Gieben Expires February 26, 2019 [Page 98] Internet-Draft Learning Go August 2018 8.6.7. Uniq Write a Go program that mimics the function of the Unix "uniq" command. This program should work as follows, given a list with the following items: 'a' 'b' 'a' 'a' 'a' 'c' 'd' 'e' 'f' 'g' it should print only those items which don't have the same successor: 'a' 'b' 'a' 'c' 'd' 'e' 'f' 'g' The next listing is a Perl implementation of the algorithm. #!/usr/bin/perl my @a = qw/a b a a a c d e f g/; print my $first = shift @a; foreach (@a) { if ($first ne $_) { print; $first = $_; } } 8.6.8. Answer The following is a "uniq" implementation in Go. package main import "fmt" func main() { list := []string{"a", "b", "a", "a", "c", "d", "e", "f"} first := list[0] fmt.Printf("%s ", first) for _, v := range list[1:] { if first != v { fmt.Printf("%s ", v) first = v } } } 8.6.9. Quine A _Quine_ is a program that prints itself. Write a Quine in Go. Gieben Expires February 26, 2019 [Page 99] Internet-Draft Learning Go August 2018 8.6.10. Answer This solution is from Russ Cox. It was posted to the Go Nuts mailing list. /* Go quine */ package main import "fmt" func main() { fmt.Printf("%s%c%s%c\n", q, 0x60, q, 0x60) } var q = `/* Go quine */ package main import "fmt" func main() { fmt.Printf("%s%c%s%c\n", q, 0x60, q, 0x60) } var q = ` 8.6.11. Processes Write a program that takes a list of all running processes and prints how many child processes each parent has spawned. The output should look like: Pid 0 has 2 children: [1 2] Pid 490 has 2 children: [1199 26524] Pid 1824 has 1 child: [7293] o For acquiring the process list, you'll need to capture the output of "ps -e -opid,ppid,comm". This output looks like: PID PPID COMMAND 9024 9023 zsh 19560 9024 ps o If a parent has one child you must print "child", if there is more than one print "children". o The process list must be numerically sorted, so you start with pid 0 and work your way up. Here is a Perl version to help you on your way (or to create complete and utter confusion). Gieben Expires February 26, 2019 [Page 100] Internet-Draft Learning Go August 2018 #!/usr/bin/perl -l my (%child, $pid, $parent); my @ps=`ps -e -opid,ppid,comm`; # capture the output from `ps` foreach (@ps[1..$#ps]) { # discard the header line ($pid, $parent, undef) = split; # split the line, discard 'comm' push @{$child{$parent}}, $pid; # save the child PIDs on a list } # Walk through the sorted PPIDs foreach (sort { $a <=> $b } keys %child) { print "Pid ", $_, " has ", @{$child{$_}}+0, " child", @{$child{$_}} == 1 ? ": " : "ren: ", "[@{$child{$_}}]"; } 8.6.12. Answer There is lots of stuff to do here. We can divide our program up in the following sections: o Starting "ps" and capturing the output. o Parsing the output and saving the child PIDs for each PPID. o Sorting the PPID list. o Printing the sorted list to the screen. In the solution presented below, we've used a "map[int][]int", i.e. a map indexed with integers, pointing to a slice of ints -- which holds the PIDs. The builtin "append" is used to grow the integer slice. A possible program is: Gieben Expires February 26, 2019 [Page 101] Internet-Draft Learning Go August 2018 package main import ( "fmt" "os/exec" "sort" "strconv" "strings" ) func main() { ps := exec.Command("ps", "-e", "-opid,ppid,comm") output, _ := ps.Output() child := make(map[int][]int) for i, s := range strings.Split(string(output), "\n") { if i == 0 { // kill first line continue } if len(s) == 0 { // kill last line continue } f := strings.Fields(s) fpp, _ := strconv.Atoi(f[1]) // parent's pid fp, _ := strconv.Atoi(f[0]) // child's pid child[fpp] = append(child[fpp], fp) } schild := make([]int, len(child)) i := 0 for k, _ := range child { schild[i] = k i++ } sort.Ints(schild) for _, ppid := range schild { fmt.Printf("Pid %d has %d child", ppid, len(child[ppid])) if len(child[ppid]) == 1 { fmt.Printf(": %v\n", child[ppid]) continue } fmt.Printf("ren: %v\n", child[ppid]) } } 8.6.13. Number cruncher o Pick six (6) random numbers from this list: "1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 25, 50, 75, 100" Numbers may be picked multiple times. Gieben Expires February 26, 2019 [Page 102] Internet-Draft Learning Go August 2018 o Pick one (1) random number ("i") in the range: "1 \ldots 1000". o Tell how, by combining the first 6 numbers (or a subset thereof) with the operators "+,-,*" and "/", you can make "i". An example. We have picked the numbers: 1, 6, 7, 8, 8 and 75. And "i" is 977. This can be done in many different ways, one way is: "((((1 * 6) * 8) + 75) * 8) - 7 = 977" or "(8*(75+(8*6)))-(7/1) = 977" Implement a number cruncher that works like that. Make it print the solution in a similar format (i.e. output should be infix with parenthesis) as used above. Calculate _all_ possible solutions and show them (or only show how many there are). In the example above there are 544 ways to do it. 8.6.14. Answer The following is one possibility. It uses recursion and backtracking to get an answer. When starting "permrec" we give 977 as the first argument: % ./permrec 977 1+(((6+7)*75)+(8/8)) = 977 #1 ... ... ((75+(8*6))*8)-7 = 977 #542 (((75+(8*6))*8)-7)*1 = 977 #543 (((75+(8*6))*8)-7)/1 = 977 #544 package main import ( "flag" "fmt" "strconv" ) const ( _ = 1000 * iota ADD SUB MUL DIV MAXPOS = 11 ) var mop = map[int]string{ADD: "+", SUB: "-", MUL: "*", DIV: "/"} Gieben Expires February 26, 2019 [Page 103] Internet-Draft Learning Go August 2018 var ( ok bool value int ) type Stack struct { i int data [MAXPOS]int } func (s *Stack) Reset() { s.i = 0 } func (s *Stack) Len() int { return s.i } func (s *Stack) Push(k int) { s.data[s.i] = k; s.i++ } func (s *Stack) Pop() int { s.i--; return s.data[s.i] } var found int var stack = new(Stack) func main() { flag.Parse() list := []int{1, 6, 7, 8, 8, 75, ADD, SUB, MUL, DIV} magic, ok := strconv.Atoi(flag.Arg(0)) // Arg0 is i if ok != nil { return } f := make([]int, MAXPOS) solve(f, list, 0, magic) } func solve(form, numberop []int, index, magic int) { var tmp int for i, v := range numberop { if v == 0 { goto NEXT } if v < ADD { // it's a number, save it tmp = numberop[i] numberop[i] = 0 } form[index] = v value, ok = rpncalc(form[0 : index+1]) if ok && value == magic { if v < ADD { numberop[i] = tmp // reset and go on } found++ fmt.Printf("%s = %d #%d\n", rpnstr(form[0:index+1]), value, found) Gieben Expires February 26, 2019 [Page 104] Internet-Draft Learning Go August 2018 } if index == MAXPOS-1 { if v < ADD { numberop[i] = tmp // reset and go on } goto NEXT } solve(form, numberop, index+1, magic) if v < ADD { numberop[i] = tmp // reset and go on } NEXT: } } func rpnstr(r []int) (ret string) { // Convert rpn to infix notation s := make([]string, 0) // Still memory intensive for k, t := range r { switch t { case ADD, SUB, MUL, DIV: var a, b string a, s = s[len(s)-1], s[:len(s)-1] b, s = s[len(s)-1], s[:len(s)-1] if k == len(r)-1 { s = append(s, b+mop[t]+a) } else { s = append(s, "("+b+mop[t]+a+")") } default: s = append(s, strconv.Itoa(t)) } } for _, v := range s { ret += v } return } func rpncalc(r []int) (int, bool) { stack.Reset() for _, t := range r { switch t { case ADD, SUB, MUL, DIV: if stack.Len() < 2 { return 0, false } a := stack.Pop() Gieben Expires February 26, 2019 [Page 105] Internet-Draft Learning Go August 2018 b := stack.Pop() if t == ADD { stack.Push(b + a) } if t == SUB { // disallow negative subresults if b-a < 0 { return 0, false } stack.Push(b - a) } if t == MUL { stack.Push(b * a) } if t == DIV { if a == 0 { return 0, false } // disallow fractions if b%a != 0 { return 0, false } stack.Push(b / a) } default: stack.Push(t) } } if stack.Len() == 1 { // there is only one! return stack.Pop(), true } return 0, false } 9. References 9.1. Informative References [bubblesort] Wikipedia, "Bubble sort", 2010. [csp] Hoare, C., "Communicating sequential processes (csp)", 1985. [duck_typing] Wikipedia, "Duck typing", 2010. Gieben Expires February 26, 2019 [Page 106] Internet-Draft Learning Go August 2018 [effective_go] Authors, G., "Effective Go", 2010. [fizzbuzz] Tech, I., "Using fizzbuzz to find developers...", 2010. [go_1_5_release_notes] Authors, G., "Go 1.5 Release Notes", 2010. [go_blog_panic] Authors, G., "Defer, panic, and recover", 2010. [go_course_day3] Pike, R., "The Go programming language, day 3", 2010. [go_nuts_interfaces] Community, G., "Function accepting a slice of interface types", 2010. [go_spec] Authors, G., "Go language specification", 2010. [go_tutorial] Authors, G., "Go tutorial", 2010. [go_web] Authors, G., "Go website", 2010. [RFC1196] Zimmerman, D., "Finger User Information Protocol", RFC 1196, DOI 10.17487/RFC1196, December 1990, . [RFC4627] Crockford, D., "The application/json Media Type for JavaScript Object Notation (JSON)", RFC 4627, DOI 10.17487/RFC4627, July 2006, . 9.2. URIs [1] https://github.com/miekg/learninggo [2] https://github.com/mmarkdown/mmark [3] https://github.com/miekg/gobook [4] http://creativecommons.org/licenses/by-nc-sa/3.0/ [5] http://www.mikespook.com/learning-go/ [6] https://github.com/mmarkdown/mmark Gieben Expires February 26, 2019 [Page 107] Internet-Draft Learning Go August 2018 [7] learninggo-2.txt [8] http://golang.org/doc/ [9] https://www.debian.org Index A array multidimensional 21 B built-in append 20, 23 cap 20 close 19 complex 20 copy 20, 23 delete 19 imag 20 len 20 make 20, 55 new 20, 55 panic 20 print 20 real 20 recover 20 C channel blocking read 85 blocking write 85 unbuffered 85 channels 6, 83 complex numbers 20 D duck typing 72 F field 58 functions as values 31 exported 44 literal 33 literals 31 Gieben Expires February 26, 2019 [Page 108] Internet-Draft Learning Go August 2018 method 29 pass-by-value 29 private 44 public 44 receiver 29 signature 32 variadic 34 G generic 75 goroutine 6, 82 I interface 71 set of methods 71 type 71 value 71 io buffered 91 io.Reader 91 K keywords break 16 continue 17 default 19 defer 32 defer list 33 else 15 fallthrough 19 for 16 go 82 goto 16 if 15 import 45 iota 12 map 23 map adding elements 24 map existence 24 map remove elements 24 package 44 range 17-18, 24 return 15 struct 57 switch 18 type 57 L Gieben Expires February 26, 2019 [Page 109] Internet-Draft Learning Go August 2018 label 16 literal composite 21, 56 M methods inherited 60 N networking Dial 93 nil 54 O operators address-of 54 and 14 bit wise xor 14 bitwise and 14 bitwise clear 14 bitwise or 14 channel 83 increment 55 not 14 or 14 P package 44 bufio 46, 49, 91 builtin 19 bytes 45 encoding/json 50 flag 50, 92 fmt 20, 49 html/template 50 io 49, 91 net/http 50 os 50 os/exec 51, 93 reflect 50, 77 ring 46 sort 50 strconv 50 sync 50 unsafe 50 R reference types 21 Gieben Expires February 26, 2019 [Page 110] Internet-Draft Learning Go August 2018 runes 13, 18 S scope local 30 slice capacity 21 length 21 structures embed 60 T tooling go 10 go build 10 go run 10 go test 47 type assertion 73 type switch 72 V variables assigning 10 declaring 10 parallel assignment 11 underscore 11 Author's Address R. (Miek) Gieben Email: miek@miek.nl Gieben Expires February 26, 2019 [Page 111]