mirror of
https://github.com/adambard/learnxinyminutes-docs.git
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444 lines
17 KiB
Go
444 lines
17 KiB
Go
---
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name: Go
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category: language
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language: Go
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filename: learngo.go
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contributors:
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- ["Sonia Keys", "https://github.com/soniakeys"]
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- ["Christopher Bess", "https://github.com/cbess"]
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- ["Jesse Johnson", "https://github.com/holocronweaver"]
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- ["Quint Guvernator", "https://github.com/qguv"]
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- ["Jose Donizetti", "https://github.com/josedonizetti"]
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- ["Alexej Friesen", "https://github.com/heyalexej"]
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- ["Clayton Walker", "https://github.com/cwalk"]
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- ["Leonid Shevtsov", "https://github.com/leonid-shevtsov"]
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---
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Go was created out of the need to get work done. It's not the latest trend
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in computer science, but it is the newest fastest way to solve real-world
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problems.
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It has familiar concepts of imperative languages with static typing.
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It's fast to compile and fast to execute, it adds easy-to-understand
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concurrency to leverage today's multi-core CPUs, and has features to
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help with large-scale programming.
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Go comes with a great standard library and an enthusiastic community.
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```go
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// Single line comment
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/* Multi-
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line comment */
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// A package clause starts every source file.
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// Main is a special name declaring an executable rather than a library.
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package main
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// Import declaration declares library packages referenced in this file.
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import (
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"fmt" // A package in the Go standard library.
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"io/ioutil" // Implements some I/O utility functions.
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m "math" // Math library with local alias m.
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"net/http" // Yes, a web server!
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"os" // OS functions like working with the file system
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"strconv" // String conversions.
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)
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// A function definition. Main is special. It is the entry point for the
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// executable program. Love it or hate it, Go uses brace brackets.
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func main() {
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// Println outputs a line to stdout.
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// Qualify it with the package name, fmt.
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fmt.Println("Hello world!")
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// Call another function within this package.
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beyondHello()
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}
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// Functions have parameters in parentheses.
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// If there are no parameters, empty parentheses are still required.
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func beyondHello() {
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var x int // Variable declaration. Variables must be declared before use.
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x = 3 // Variable assignment.
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// "Short" declarations use := to infer the type, declare, and assign.
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y := 4
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sum, prod := learnMultiple(x, y) // Function returns two values.
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fmt.Println("sum:", sum, "prod:", prod) // Simple output.
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learnTypes() // < y minutes, learn more!
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}
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/* <- multiline comment
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Functions can have parameters and (multiple!) return values.
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Here `x`, `y` are the arguments and `sum`, `prod` is the signature (what's returned).
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Note that `x` and `sum` receive the type `int`.
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*/
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func learnMultiple(x, y int) (sum, prod int) {
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return x + y, x * y // Return two values.
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}
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// Some built-in types and literals.
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func learnTypes() {
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// Short declaration usually gives you what you want.
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str := "Learn Go!" // string type.
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s2 := `A "raw" string literal
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can include line breaks.` // Same string type.
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// Non-ASCII literal. Go source is UTF-8.
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g := 'Σ' // rune type, an alias for int32, holds a unicode code point.
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f := 3.14195 // float64, an IEEE-754 64-bit floating point number.
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c := 3 + 4i // complex128, represented internally with two float64's.
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// var syntax with initializers.
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var u uint = 7 // Unsigned, but implementation dependent size as with int.
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var pi float32 = 22. / 7
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// Conversion syntax with a short declaration.
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n := byte('\n') // byte is an alias for uint8.
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// Arrays have size fixed at compile time.
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var a4 [4]int // An array of 4 ints, initialized to all 0.
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a5 := [...]int{3, 1, 5, 10, 100} // An array initialized with a fixed size of five
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// elements, with values 3, 1, 5, 10, and 100.
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// Slices have dynamic size. Arrays and slices each have advantages
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// but use cases for slices are much more common.
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s3 := []int{4, 5, 9} // Compare to a5. No ellipsis here.
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s4 := make([]int, 4) // Allocates slice of 4 ints, initialized to all 0.
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var d2 [][]float64 // Declaration only, nothing allocated here.
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bs := []byte("a slice") // Type conversion syntax.
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// Because they are dynamic, slices can be appended to on-demand.
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// To append elements to a slice, the built-in append() function is used.
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// First argument is a slice to which we are appending. Commonly,
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// the array variable is updated in place, as in example below.
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s := []int{1, 2, 3} // Result is a slice of length 3.
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s = append(s, 4, 5, 6) // Added 3 elements. Slice now has length of 6.
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fmt.Println(s) // Updated slice is now [1 2 3 4 5 6]
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// To append another slice, instead of list of atomic elements we can
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// pass a reference to a slice or a slice literal like this, with a
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// trailing ellipsis, meaning take a slice and unpack its elements,
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// appending them to slice s.
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s = append(s, []int{7, 8, 9}...) // Second argument is a slice literal.
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fmt.Println(s) // Updated slice is now [1 2 3 4 5 6 7 8 9]
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p, q := learnMemory() // Declares p, q to be type pointer to int.
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fmt.Println(*p, *q) // * follows a pointer. This prints two ints.
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// Maps are a dynamically growable associative array type, like the
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// hash or dictionary types of some other languages.
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m := map[string]int{"three": 3, "four": 4}
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m["one"] = 1
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// Unused variables are an error in Go.
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// The underscore lets you "use" a variable but discard its value.
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_, _, _, _, _, _, _, _, _, _ = str, s2, g, f, u, pi, n, a5, s4, bs
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// Usually you use it to ignore one of the return values of a function
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// For example, in a quick and dirty script you might ignore the
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// error value returned from os.Create, and expect that the file
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// will always be created.
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file, _ := os.Create("output.txt")
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fmt.Fprint(file, "This is how you write to a file, by the way")
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file.Close()
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// Output of course counts as using a variable.
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fmt.Println(s, c, a4, s3, d2, m)
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learnFlowControl() // Back in the flow.
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}
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// It is possible, unlike in many other languages for functions in go
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// to have named return values.
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// Assigning a name to the type being returned in the function declaration line
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// allows us to easily return from multiple points in a function as well as to
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// only use the return keyword, without anything further.
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func learnNamedReturns(x, y int) (z int) {
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z = x * y
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return // z is implicit here, because we named it earlier.
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}
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// Go is fully garbage collected. It has pointers but no pointer arithmetic.
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// You can make a mistake with a nil pointer, but not by incrementing a pointer.
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func learnMemory() (p, q *int) {
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// Named return values p and q have type pointer to int.
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p = new(int) // Built-in function new allocates memory.
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// The allocated int is initialized to 0, p is no longer nil.
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s := make([]int, 20) // Allocate 20 ints as a single block of memory.
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s[3] = 7 // Assign one of them.
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r := -2 // Declare another local variable.
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return &s[3], &r // & takes the address of an object.
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}
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func expensiveComputation() float64 {
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return m.Exp(10)
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}
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func learnFlowControl() {
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// If statements require brace brackets, and do not require parentheses.
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if true {
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fmt.Println("told ya")
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}
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// Formatting is standardized by the command line command "go fmt".
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if false {
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// Pout.
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} else {
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// Gloat.
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}
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// Use switch in preference to chained if statements.
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x := 42.0
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switch x {
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case 0:
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case 1:
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case 42:
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// Cases don't "fall through".
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/*
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There is a `fallthrough` keyword however, see:
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https://github.com/golang/go/wiki/Switch#fall-through
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*/
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case 43:
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// Unreached.
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default:
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// Default case is optional.
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}
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// Like if, for doesn't use parens either.
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// Variables declared in for and if are local to their scope.
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for x := 0; x < 3; x++ { // ++ is a statement.
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fmt.Println("iteration", x)
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}
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// x == 42 here.
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// For is the only loop statement in Go, but it has alternate forms.
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for { // Infinite loop.
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break // Just kidding.
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continue // Unreached.
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}
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// You can use range to iterate over an array, a slice, a string, a map, or a channel.
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// range returns one (channel) or two values (array, slice, string and map).
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for key, value := range map[string]int{"one": 1, "two": 2, "three": 3} {
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// for each pair in the map, print key and value
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fmt.Printf("key=%s, value=%d\n", key, value)
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}
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// If you only need the value, use the underscore as the key
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for _, name := range []string{"Bob", "Bill", "Joe"} {
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fmt.Printf("Hello, %s\n", name)
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}
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// As with for, := in an if statement means to declare and assign
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// y first, then test y > x.
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if y := expensiveComputation(); y > x {
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x = y
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}
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// Function literals are closures.
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xBig := func() bool {
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return x > 10000 // References x declared above switch statement.
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}
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x = 99999
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fmt.Println("xBig:", xBig()) // true
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x = 1.3e3 // This makes x == 1300
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fmt.Println("xBig:", xBig()) // false now.
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// What's more is function literals may be defined and called inline,
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// acting as an argument to function, as long as:
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// a) function literal is called immediately (),
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// b) result type matches expected type of argument.
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fmt.Println("Add + double two numbers: ",
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func(a, b int) int {
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return (a + b) * 2
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}(10, 2)) // Called with args 10 and 2
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// => Add + double two numbers: 24
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// When you need it, you'll love it.
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goto love
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love:
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learnFunctionFactory() // func returning func is fun(3)(3)
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learnDefer() // A quick detour to an important keyword.
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learnInterfaces() // Good stuff coming up!
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}
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func learnFunctionFactory() {
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// Next two are equivalent, with second being more practical
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fmt.Println(sentenceFactory("summer")("A beautiful", "day!"))
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d := sentenceFactory("summer")
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fmt.Println(d("A beautiful", "day!"))
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fmt.Println(d("A lazy", "afternoon!"))
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}
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// Decorators are common in other languages. Same can be done in Go
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// with function literals that accept arguments.
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func sentenceFactory(mystring string) func(before, after string) string {
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return func(before, after string) string {
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return fmt.Sprintf("%s %s %s", before, mystring, after) // new string
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}
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}
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func learnDefer() (ok bool) {
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// Deferred statements are executed just before the function returns.
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defer fmt.Println("deferred statements execute in reverse (LIFO) order.")
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defer fmt.Println("\nThis line is being printed first because")
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// Defer is commonly used to close a file, so the function closing the
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// file stays close to the function opening the file.
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return true
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}
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// Define Stringer as an interface type with one method, String.
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type Stringer interface {
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String() string
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}
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// Define pair as a struct with two fields, ints named x and y.
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type pair struct {
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x, y int
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}
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// Define a method on type pair. Pair now implements Stringer because Pair has defined all the methods in the interface.
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func (p pair) String() string { // p is called the "receiver"
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// Sprintf is another public function in package fmt.
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// Dot syntax references fields of p.
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return fmt.Sprintf("(%d, %d)", p.x, p.y)
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}
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func learnInterfaces() {
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// Brace syntax is a "struct literal". It evaluates to an initialized
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// struct. The := syntax declares and initializes p to this struct.
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p := pair{3, 4}
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fmt.Println(p.String()) // Call String method of p, of type pair.
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var i Stringer // Declare i of interface type Stringer.
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i = p // Valid because pair implements Stringer
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// Call String method of i, of type Stringer. Output same as above.
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fmt.Println(i.String())
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// Functions in the fmt package call the String method to ask an object
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// for a printable representation of itself.
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fmt.Println(p) // Output same as above. Println calls String method.
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fmt.Println(i) // Output same as above.
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learnVariadicParams("great", "learning", "here!")
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}
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// Functions can have variadic parameters.
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func learnVariadicParams(myStrings ...interface{}) {
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// Iterate each value of the variadic.
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// The underbar here is ignoring the index argument of the array.
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for _, param := range myStrings {
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fmt.Println("param:", param)
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}
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// Pass variadic value as a variadic parameter.
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fmt.Println("params:", fmt.Sprintln(myStrings...))
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learnErrorHandling()
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}
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func learnErrorHandling() {
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// ", ok" idiom used to tell if something worked or not.
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m := map[int]string{3: "three", 4: "four"}
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if x, ok := m[1]; !ok { // ok will be false because 1 is not in the map.
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fmt.Println("no one there")
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} else {
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fmt.Print(x) // x would be the value, if it were in the map.
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}
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// An error value communicates not just "ok" but more about the problem.
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if _, err := strconv.Atoi("non-int"); err != nil { // _ discards value
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// prints 'strconv.ParseInt: parsing "non-int": invalid syntax'
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fmt.Println(err)
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}
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// We'll revisit interfaces a little later. Meanwhile,
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learnConcurrency()
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}
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// c is a channel, a concurrency-safe communication object.
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func inc(i int, c chan int) {
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c <- i + 1 // <- is the "send" operator when a channel appears on the left.
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}
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// We'll use inc to increment some numbers concurrently.
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func learnConcurrency() {
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// Same make function used earlier to make a slice. Make allocates and
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// initializes slices, maps, and channels.
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c := make(chan int)
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// Start three concurrent goroutines. Numbers will be incremented
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// concurrently, perhaps in parallel if the machine is capable and
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// properly configured. All three send to the same channel.
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go inc(0, c) // go is a statement that starts a new goroutine.
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go inc(10, c)
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go inc(-805, c)
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// Read three results from the channel and print them out.
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// There is no telling in what order the results will arrive!
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fmt.Println(<-c, <-c, <-c) // channel on right, <- is "receive" operator.
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cs := make(chan string) // Another channel, this one handles strings.
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ccs := make(chan chan string) // A channel of string channels.
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go func() { c <- 84 }() // Start a new goroutine just to send a value.
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go func() { cs <- "wordy" }() // Again, for cs this time.
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// Select has syntax like a switch statement but each case involves
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// a channel operation. It selects a case at random out of the cases
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// that are ready to communicate.
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select {
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case i := <-c: // The value received can be assigned to a variable,
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fmt.Printf("it's a %T", i)
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case <-cs: // or the value received can be discarded.
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fmt.Println("it's a string")
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case <-ccs: // Empty channel, not ready for communication.
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fmt.Println("didn't happen.")
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}
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// At this point a value was taken from either c or cs. One of the two
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// goroutines started above has completed, the other will remain blocked.
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learnWebProgramming() // Go does it. You want to do it too.
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}
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// A single function from package http starts a web server.
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func learnWebProgramming() {
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// First parameter of ListenAndServe is TCP address to listen to.
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// Second parameter is an interface, specifically http.Handler.
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go func() {
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err := http.ListenAndServe(":8080", pair{})
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fmt.Println(err) // don't ignore errors
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}()
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requestServer()
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}
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// Make pair an http.Handler by implementing its only method, ServeHTTP.
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func (p pair) ServeHTTP(w http.ResponseWriter, r *http.Request) {
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// Serve data with a method of http.ResponseWriter.
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w.Write([]byte("You learned Go in Y minutes!"))
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}
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func requestServer() {
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resp, err := http.Get("http://localhost:8080")
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fmt.Println(err)
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defer resp.Body.Close()
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body, err := ioutil.ReadAll(resp.Body)
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fmt.Printf("\nWebserver said: `%s`", string(body))
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}
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```
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## Further Reading
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The root of all things Go is the [official Go web site](http://golang.org/).
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There you can follow the tutorial, play interactively, and read lots.
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Aside from a tour, [the docs](https://golang.org/doc/) contain information on
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how to write clean and effective Go code, package and command docs, and release history.
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The language definition itself is highly recommended. It's easy to read
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and amazingly short (as language definitions go these days.)
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You can play around with the code on [Go playground](https://play.golang.org/p/tnWMjr16Mm). Try to change it and run it from your browser! Note that you can use [https://play.golang.org](https://play.golang.org) as a [REPL](https://en.wikipedia.org/wiki/Read-eval-print_loop) to test things and code in your browser, without even installing Go.
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On the reading list for students of Go is the [source code to the standard
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library](http://golang.org/src/pkg/). Comprehensively documented, it
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demonstrates the best of readable and understandable Go, Go style, and Go
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idioms. Or you can click on a function name in [the
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documentation](http://golang.org/pkg/) and the source code comes up!
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Another great resource to learn Go is [Go by example](https://gobyexample.com/).
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Go Mobile adds support for mobile platforms (Android and iOS). You can write all-Go native mobile apps or write a library that contains bindings from a Go package, which can be invoked via Java (Android) and Objective-C (iOS). Check out the [Go Mobile page](https://github.com/golang/go/wiki/Mobile) for more information.
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