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Adam Bard
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---
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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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---
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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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"net/http" // Yes, a web server!
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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 parens 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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// Functions can have parameters and (multiple!) return values.
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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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s := "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 uint32, holds a UTF-8 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 float64s
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// Var syntax with an 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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a3 := [...]int{3, 1, 5} // an array of 3 ints, initialized as shown
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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 a3. 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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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 underbar lets you "use" a variable but discard its value.
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_, _, _, _, _, _, _, _, _ = s2, g, f, u, pi, n, a3, s4, bs
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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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// 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() int {
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return 1e6
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}
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func learnFlowControl() {
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// If statements require brace brackets, and do not require parens.
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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 := 1
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switch x {
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case 0:
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case 1:
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// cases don't "fall through"
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case 2:
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// unreached
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}
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// Like if, for doesn't use parens either.
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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 == 1 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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// As with for, := in an if statement means to declare and assign y first,
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// 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 > 100 // references x declared above switch statement.
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}
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fmt.Println("xBig:", xBig()) // true (we last assigned 1e6 to x)
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x /= 1e5 // this makes it == 10
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fmt.Println("xBig:", xBig()) // false now
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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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learnInterfaces() // Good stuff coming up!
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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.
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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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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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cc := make(chan chan string) // a channel of 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.Println("it's a", 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 <-cc: // 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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// ListenAndServe first parameter is TCP address to listen at.
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// Second parameter is an interface, specifically http.Handler.
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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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// 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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```
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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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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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On the reading list for students of Go is the source code to the standard
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library. Comprehensively documented, it demonstrates the best of readable
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and understandable Go, Go style, and Go idioms. Click on a function name
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in the documentation and the source code comes up!
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