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Go Tutorial

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1. Basics

1.1 Packages, variables, and functions

Learn the basic components of any Go programLearn the basic components of any Go program..

1.1.1 Packages

By convention, the package name is the same as the last element of the import path. For instance, the “math/rand” package comprises files that begin with the statement package rand.


package main

import (
	"fmt"
	"math/rand"
)

func main() {
	fmt.Println("My favorite number is", rand.Intn(10))
}

1.1.2. Imports


import "fmt"
import "math"

or


import (
	"fmt"
	"math"
)

1.1.3. Exported names

In Go, a name is exported if it begins with a capital letter. For example, Pizza is an exported name, as is Pi, which is exported from the math package.

math.Pi

1.1.4. Functions

package main

import "fmt"

func add(x int, y int) int {
	return x + y
}

func main() {
	fmt.Println(add(42, 13))
}

When two or more consecutive named function parameters share a type, you can omit the type from all but the last.

In this example, we shortened

x int, y int

to

x, y int

1.1.5. Multiple results

A function can return any number of results.

package main

import "fmt"

func swap(x, y string) (string, string) {
	return y, x
}

func main() {
	a, b := swap("hello", "world")
	fmt.Println(a, b)
}

1.1.6 Named return values

Go’s return values may be named. If so, they are treated as variables defined at the top of the function.

These names should be used to document the meaning of the return values.


package main

import "fmt"

func split(sum int) (x, y int) {
	x = sum * 4 / 9
	y = sum - x
	return
}

func main() {
	fmt.Println(split(17))
}

1.1.7 Variables

  • The var statement declares a list of variables; as in function argument lists, the type is last. A var statement can be at package or function level. We see both in this example.

package main

import "fmt"

var c, python, java bool

func main() {
	var i int
	fmt.Println(i, c, python, java)
}
  • A var declaration can include initializers, one per variable. If an initializer is present, the type can be omitted; the variable will take the type of the initializer.

package main

import "fmt"

var i, j int = 1, 2

func main() {
	var c, python, java = true, false, "no!"
	fmt.Println(i, j, c, python, java)
}
  • Inside a function, the := short assignment statement can be used in place of a var declaration with implicit type. Outside a function, every statement begins with a keyword (var, func, and so on) and so the := construct is not available.

package main

import "fmt"

func main() {
	var i, j int = 1, 2
	k := 3
	c, python, java := true, false, "no!"

	fmt.Println(i, j, k, c, python, java)
}

1.1.8 Basic types

Go’s basic types are


bool

string

int  int8  int16  int32  int64
uint uint8 uint16 uint32 uint64 uintptr

byte // alias for uint8

rune // alias for int32
     // represents a Unicode code point

float32 float64

complex64 complex128



package main

import (
	"fmt"
	"math/cmplx"
)

var (
	ToBe   bool       = false
	MaxInt uint64     = 1<<64 - 1
	z      complex128 = cmplx.Sqrt(-5 + 12i)
)

func main() {
	fmt.Printf("Type: %T Value: %v\n", ToBe, ToBe)
	fmt.Printf("Type: %T Value: %v\n", MaxInt, MaxInt)
	fmt.Printf("Type: %T Value: %v\n", z, z)
}

1.1.9 Constants

Constants are declared like variables, but with the const keyword.

Constants can be character, string, boolean, or numeric values.

Constants cannot be declared using the := syntax.

package main

import "fmt"

const Pi = 3.14

func main() {
	const World = "世界"
	fmt.Println("Hello", World)
	fmt.Println("Happy", Pi, "Day")

	const Truth = true
	fmt.Println("Go rules?", Truth)
}

1.2 Flow control statements: for, if, else, switch and defer

Learn how to control the flow of your code with conditionals, loops, switches and defers.

1.2.1 For

Go has only one looping construct, the for loop.


package main

import "fmt"

func main() {
	sum := 0
	for i := 0; i < 10; i++ {
		sum += i
	}
	fmt.Println(sum)
}

The init and post statements are optional.


package main

import "fmt"

func main() {
	sum := 1
	for ; sum < 1000; {
		sum += sum
	}
	fmt.Println(sum)
}

For is Go’s “while”


package main

import "fmt"

func main() {
	sum := 1
	for sum < 1000 {
		sum += sum
	}
	fmt.Println(sum)
}

Forever


package main

func main() {
	for {
	}
}

1.2.2 If


package main

import (
	"fmt"
	"math"
)

func sqrt(x float64) string {
	if x < 0 {
		return sqrt(-x) + "i"
	}
	return fmt.Sprint(math.Sqrt(x))
}

func main() {
	fmt.Println(sqrt(2), sqrt(-4))
}

If with a short statement

Like for, the if statement can start with a short statement to execute before the condition. Variables declared by the statement are only in scope until the end of the if.


package main

import (
	"fmt"
	"math"
)

func pow(x, n, lim float64) float64 {
	if v := math.Pow(x, n); v < lim {
		return v
	}
	return lim
}

func main() {
	fmt.Println(
		pow(3, 2, 10),
		pow(3, 3, 20),
	)
}

1.2.3 If and else


package main

import (
	"fmt"
	"math"
)

func pow(x, n, lim float64) float64 {
	if v := math.Pow(x, n); v < lim {
		return v
	} else {
		fmt.Printf("%g >= %g\n", v, lim)
	}
	// can't use v here, though
	return lim
}

func main() {
	fmt.Println(
		pow(3, 2, 10),
		pow(3, 3, 20),
	)
}

1.2.4 Switch

package main

import (
	"fmt"
	"runtime"
)

func main() {
	fmt.Print("Go runs on ")
	switch os := runtime.GOOS; os {
	case "darwin":
		fmt.Println("OS X.")
	case "linux":
		fmt.Println("Linux.")
	default:
		// freebsd, openbsd,
		// plan9, windows...
		fmt.Printf("%s.\n", os)
	}
}

Switch cases evaluate cases from top to bottom, stopping when a case succeeds.

(For example,


switch i {
case 0:
case f():
}

does not call f if i==0.)

Switch without a condition is the same as switch true.

package main

import (
	"fmt"
	"time"
)

func main() {
	t := time.Now()
	switch {
	case t.Hour() < 12:
		fmt.Println("Good morning!")
	case t.Hour() < 17:
		fmt.Println("Good afternoon.")
	default:
		fmt.Println("Good evening.")
	}
}

1.2.5 Defer

A defer statement defers the execution of a function until the surrounding function returns.

The deferred call’s arguments are evaluated immediately, but the function call is not executed until the surrounding function returns.


package main

import "fmt"

func main() {
	defer fmt.Println("world")

	fmt.Println("hello")
}

Deferred function calls are pushed onto a stack. When a function returns, its deferred calls are executed in last-in-first-out order.


package main

import "fmt"

func main() {
	fmt.Println("counting")

	for i := 0; i < 10; i++ {
		defer fmt.Println(i)
	}

	fmt.Println("done")
}

1.3 More types: structs, slices, and maps.

Learn how to define types based on existing ones: this lesson covers structs, arrays, slices, and maps.

1.3.1 Pointers

The type *T is a pointer to a T value. Its zero value is nil.

var p *int

The & operator generates a pointer to its operand.

i := 42
p = &i

package main

import "fmt"

func main() {
	i, j := 42, 2701

	p := &i         // point to i
	fmt.Println(*p) // read i through the pointer
	*p = 21         // set i through the pointer
	fmt.Println(i)  // see the new value of i

	p = &j         // point to j
	*p = *p / 37   // divide j through the pointer
	fmt.Println(j) // see the new value of j
}

1.3.2 Structs

package main

import "fmt"

type Vertex struct {
	X int
	Y int
}

func main() {
	fmt.Println(Vertex{1, 2})
}

package main

import "fmt"

type Vertex struct {
	X int
	Y int
}

func main() {
	v := Vertex{1, 2}
	v.X = 4
	fmt.Println(v.X)
}

Struct fields can be accessed through a struct pointer.

To access the field X of a struct when we have the struct pointer p we could write (*p).X. However, that notation is cumbersome, so the language permits us instead to write just p.X, without the explicit dereference.

package main

import "fmt"

type Vertex struct {
	X int
	Y int
}

func main() {
	v := Vertex{1, 2}
	p := &v
	p.X = 1e9
	fmt.Println(v)
}

A struct literal denotes a newly allocated struct value by listing the values of its fields.

You can list just a subset of fields by using the Name: syntax. (And the order of named fields is irrelevant.)

The special prefix & returns a pointer to the struct value.

package main

import "fmt"

type Vertex struct {
	X, Y int
}

var (
	v1 = Vertex{1, 2}  // has type Vertex
	v2 = Vertex{X: 1}  // Y:0 is implicit
	v3 = Vertex{}      // X:0 and Y:0
	p  = &Vertex{1, 2} // has type *Vertex
)

func main() {
	fmt.Println(v1, p, v2, v3)
}

1.3.3 Arrays

The type [n]T is an array of n values of type T.

The expression


var a [10]int

declares a variable a as an array of ten integers.

An array’s length is part of its type, so arrays cannot be resized. This seems limiting, but don’t worry; Go provides a convenient way of working with arrays.

package main

import "fmt"

func main() {
	var a [2]string
	a[0] = "Hello"
	a[1] = "World"
	fmt.Println(a[0], a[1])
	fmt.Println(a)

	primes := [6]int{2, 3, 5, 7, 11, 13}
	fmt.Println(primes)
}

1.3.4 Slices

An array has a fixed size. A slice, on the other hand, is a dynamically-sized, flexible view into the elements of an array. In practice, slices are much more common than arrays.

The following expression creates a slice which includes elements 1 through 3 of a:

a[1:4]


package main

import "fmt"

func main() {
	primes := [6]int{2, 3, 5, 7, 11, 13}

	var s []int = primes[1:4]
	fmt.Println(s)
}

A slice does not store any data, it just describes a section of an underlying array.


import "fmt"

func main() {
	names := [4]string{
		"John",
		"Paul",
		"George",
		"Ringo",
	}
	fmt.Println(names)

	a := names[0:2]
	b := names[1:3]
	fmt.Println(a, b)

	b[0] = "XXX"
	fmt.Println(a, b)
	fmt.Println(names)
}

A slice literal is like an array literal without the length.

This is an array literal:

[3]bool{true, true, false}

And this creates the same array as above, then builds a slice that references it:

[]bool{true, true, false}


package main

import "fmt"

func main() {
	q := []int{2, 3, 5, 7, 11, 13}
	fmt.Println(q)

	r := []bool{true, false, true, true, false, true}
	fmt.Println(r)

	s := []struct {
		i int
		b bool
	}{
		{2, true},
		{3, false},
		{5, true},
		{7, true},
		{11, false},
		{13, true},
	}
	fmt.Println(s)
}

When slicing, you may omit the high or low bounds to use their defaults instead. The default is zero for the low bound and the length of the slice for the high bound.

For the array

var a [10]int

these slice expressions are equivalent:

a[0:10]
a[:10]
a[0:]
a[:]


package main

import "fmt"

func main() {
	s := []int{2, 3, 5, 7, 11, 13}

	s = s[1:4]
	fmt.Println(s)

	s = s[:2]
	fmt.Println(s)

	s = s[1:]
	fmt.Println(s)
}

A slice has both a length and a capacity.

The length of a slice is the number of elements it contains.

The capacity of a slice is the number of elements in the underlying array, counting from the first element in the slice.

The length and capacity of a slice s can be obtained using the expressions len(s) and cap(s).


package main

import "fmt"

func main() {
	s := []int{2, 3, 5, 7, 11, 13}
	printSlice(s)

	// Slice the slice to give it zero length.
	s = s[:0]
	printSlice(s)

	// Extend its length.
	s = s[:4]
	printSlice(s)

	// Drop its first two values.
	s = s[2:]
	printSlice(s)
}

func printSlice(s []int) {
	fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}

The zero value of a slice is nil.

A nil slice has a length and capacity of 0 and has no underlying array.


package main

import "fmt"

func main() {
	var s []int
	fmt.Println(s, len(s), cap(s))
	if s == nil {
		fmt.Println("nil!")
	}
}

Creating a slice with make

Slices can be created with the built-in make function; this is how you create dynamically-sized arrays.

The make function allocates a zeroed array and returns a slice that refers to that array:

a := make([]int, 5)  // len(a)=5

To specify a capacity, pass a third argument to make:

b := make([]int, 0, 5) // len(b)=0, cap(b)=5
b = b[:cap(b)] // len(b)=5, cap(b)=5
b = b[1:]      // len(b)=4, cap(b)=4


package main

import "fmt"

func main() {
	a := make([]int, 5)
	printSlice("a", a)

	b := make([]int, 0, 5)
	printSlice("b", b)

	c := b[:2]
	printSlice("c", c)

	d := c[2:5]
	printSlice("d", d)
}

func printSlice(s string, x []int) {
	fmt.Printf("%s len=%d cap=%d %v\n",
		s, len(x), cap(x), x)
}

Slices can contain any type, including other slices.


package main

import (
	"fmt"
	"strings"
)

func main() {
	// Create a tic-tac-toe board.
	board := [][]string{
		[]string{"_", "_", "_"},
		[]string{"_", "_", "_"},
		[]string{"_", "_", "_"},
	}

	// The players take turns.
	board[0][0] = "X"
	board[2][2] = "O"
	board[1][2] = "X"
	board[1][0] = "O"
	board[0][2] = "X"

	for i := 0; i < len(board); i++ {
		fmt.Printf("%s\n", strings.Join(board[i], " "))
	}
}

Appending to a slice


package main

import "fmt"

func main() {
	var s []int
	printSlice(s)

	// append works on nil slices.
	s = append(s, 0)
	printSlice(s)

	// The slice grows as needed.
	s = append(s, 1)
	printSlice(s)

	// We can add more than one element at a time.
	s = append(s, 2, 3, 4)
	printSlice(s)
}

func printSlice(s []int) {
	fmt.Printf("len=%d cap=%d %v\n", len(s), cap(s), s)
}

1.3.5 Range

The range form of the for loop iterates over a slice or map.

When ranging over a slice, two values are returned for each iteration. The first is the index, and the second is a copy of the element at that index.


package main

import "fmt"

var pow = []int{1, 2, 4, 8, 16, 32, 64, 128}

func main() {
	for i, v := range pow {
		fmt.Printf("2**%d = %d\n", i, v)
	}
}

You can skip the index or value by assigning to _.


for i, \_ := range pow
for \_, value := range pow

If you only want the index, you can omit the second variable.


for i := range pow



package main

import "fmt"

func main() {
	pow := make([]int, 10)
	for i := range pow {
		pow[i] = 1 << uint(i) // == 2**i
	}
	for _, value := range pow {
		fmt.Printf("%d\n", value)
	}
}

1.3.6 Maps

A map maps keys to values.

The zero value of a map is nil. A nil map has no keys, nor can keys be added.

The make function returns a map of the given type, initialized and ready for use.


package main

import "fmt"

type Vertex struct {
	Lat, Long float64
}

var m map[string]Vertex

func main() {
	m = make(map[string]Vertex)
	m["Bell Labs"] = Vertex{
		40.68433, -74.39967,
	}
	fmt.Println(m["Bell Labs"])
}

Map literals are like struct literals, but the keys are required.


package main

import "fmt"

type Vertex struct {
	Lat, Long float64
}

var m = map[string]Vertex{
	"Bell Labs": Vertex{
		40.68433, -74.39967,
	},
	"Google": Vertex{
		37.42202, -122.08408,
	},
}

func main() {
	fmt.Println(m)
}



package main

import "fmt"

type Vertex struct {
	Lat, Long float64
}

var m = map[string]Vertex{
	"Bell Labs": {40.68433, -74.39967},
	"Google":    {37.42202, -122.08408},
}

func main() {
	fmt.Println(m)
}

mutating maps


package main

import "fmt"

func main() {
	m := make(map[string]int)

	m["Answer"] = 42
	fmt.Println("The value:", m["Answer"])

	m["Answer"] = 48
	fmt.Println("The value:", m["Answer"])

	delete(m, "Answer")
	fmt.Println("The value:", m["Answer"])

	v, ok := m["Answer"]
	fmt.Println("The value:", v, "Present?", ok)
}

1.3.7 Function values

Functions are values too. They can be passed around just like other values.

Function values may be used as function arguments and return values.


package main

import (
	"fmt"
	"math"
)

func compute(fn func(float64, float64) float64) float64 {
	return fn(3, 4)
}

func main() {
	hypot := func(x, y float64) float64 {
		return math.Sqrt(x*x + y*y)
	}
	fmt.Println(hypot(5, 12))

	fmt.Println(compute(hypot))
	fmt.Println(compute(math.Pow))
}

Function closures

Go functions may be closures. A closure is a function value that references variables from outside its body. The function may access and assign to the referenced variables; in this sense the function is “bound” to the variables.

For example, the adder function returns a closure. Each closure is bound to its own sum variable.


package main

import "fmt"

func adder() func(int) int {
	sum := 0
	return func(x int) int {
		sum += x
		return sum
	}
}

func main() {
	pos, neg := adder(), adder()
	for i := 0; i < 10; i++ {
		fmt.Println(
			pos(i),
			neg(-2*i),
		)
	}
}

2. Methods and interfaces

2.1 Methods

Go does not have classes. However, you can define methods on types.

A method is a function with a special receiver argument.

The receiver appears in its own argument list between the func keyword and the method name.


package main

import (
	"fmt"
	"math"
)

type Vertex struct {
	X, Y float64
}

func (v Vertex) Abs() float64 {
	return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

func main() {
	v := Vertex{3, 4}
	fmt.Println(v.Abs())
}

Remember: a method is just a function with a receiver argument.


package main

import (
	"fmt"
	"math"
)

type Vertex struct {
	X, Y float64
}

func Abs(v Vertex) float64 {
	return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

func main() {
	v := Vertex{3, 4}
	fmt.Println(Abs(v))
}

You can declare a method on non-struct types, too.

You can only declare a method with a receiver whose type is defined in the same package as the method. You cannot declare a method with a receiver whose type is defined in another package (which includes the built-in types such as int).


package main

import (
	"fmt"
	"math"
)

type MyFloat float64

func (f MyFloat) Abs() float64 {
	if f < 0 {
		return float64(-f)
	}
	return float64(f)
}

func main() {
	f := MyFloat(-math.Sqrt2)
	fmt.Println(f.Abs())
}

You can declare methods with pointer receivers.

This means the receiver type has the literal syntax *T for some type T. (Also, T cannot itself be a pointer such as *int.)

Methods with pointer receivers can modify the value to which the receiver points (as Scale does here). Since methods often need to modify their receiver, pointer receivers are more common than value receivers.


package main

import (
	"fmt"
	"math"
)

type Vertex struct {
	X, Y float64
}

func (v Vertex) Abs() float64 {
	return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

func (v *Vertex) Scale(f float64) {
	v.X = v.X * f
	v.Y = v.Y * f
}

func main() {
	v := Vertex{3, 4}
	v.Scale(10)
	fmt.Println(v.Abs())
}

Methods with pointer receivers take either a value or a pointer as the receiver when they are called:


var v Vertex
v.Scale(5)  // OK
p := &v
p.Scale(10) // OK

For the statement v.Scale(5), even though v is a value and not a pointer, the method with the pointer receiver is called automatically. That is, as a convenience, Go interprets the statement v.Scale(5) as (&v).Scale(5) since the Scale method has a pointer receiver.


package main

import "fmt"

type Vertex struct {
	X, Y float64
}

func (v *Vertex) Scale(f float64) {
	v.X = v.X * f
	v.Y = v.Y * f
}

func ScaleFunc(v *Vertex, f float64) {
	v.X = v.X * f
	v.Y = v.Y * f
}

func main() {
	v := Vertex{3, 4}
	v.Scale(2)
	ScaleFunc(&v, 10)

	p := &Vertex{4, 3}
	p.Scale(3)
	ScaleFunc(p, 8)

	fmt.Println(v, p)
}

Methods with value receivers take either a value or a pointer as the receiver when they are called:


var v Vertex
fmt.Println(v.Abs()) // OK
p := &v
fmt.Println(p.Abs()) // OK

In this case, the method call p.Abs() is interpreted as (*p).Abs().


package main

import (
	"fmt"
	"math"
)

type Vertex struct {
	X, Y float64
}

func (v Vertex) Abs() float64 {
	return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

func AbsFunc(v Vertex) float64 {
	return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

func main() {
	v := Vertex{3, 4}
	fmt.Println(v.Abs())
	fmt.Println(AbsFunc(v))

	p := &Vertex{4, 3}
	fmt.Println(p.Abs())
	fmt.Println(AbsFunc(*p))
}

Choosing a value or pointer receiver

There are two reasons to use a pointer receiver.

The first is so that the method can modify the value that its receiver points to.

The second is to avoid copying the value on each method call. This can be more efficient if the receiver is a large struct.

In general, all methods on a given type should have either value or pointer receivers, but not a mixture of both.


package main

import (
	"fmt"
	"math"
)

type Vertex struct {
	X, Y float64
}

func (v *Vertex) Scale(f float64) {
	v.X = v.X * f
	v.Y = v.Y * f
}

func (v *Vertex) Abs() float64 {
	return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

func main() {
	v := &Vertex{3, 4}
	fmt.Printf("Before scaling: %+v, Abs: %v\n", v, v.Abs())
	v.Scale(5)
	fmt.Printf("After scaling: %+v, Abs: %v\n", v, v.Abs())
}

2.2 Interfaces

An interface type is defined as a set of method signatures.

A value of interface type can hold any value that implements those methods.

Note: There is an error in the example code on line 22. Vertex (the value type) doesn’t implement Abser because the Abs method is defined only on *Vertex (the pointer type).


package main

import (
	"fmt"
	"math"
)

type Abser interface {
	Abs() float64
}

func main() {
	var a Abser
	f := MyFloat(-math.Sqrt2)
	v := Vertex{3, 4}

	a = f  // a MyFloat implements Abser
	a = &v // a *Vertex implements Abser

	// In the following line, v is a Vertex (not *Vertex)
	// and does NOT implement Abser.
	//a = v

	fmt.Println(a.Abs())
}

type MyFloat float64

func (f MyFloat) Abs() float64 {
	if f < 0 {
		return float64(-f)
	}
	return float64(f)
}

type Vertex struct {
	X, Y float64
}

func (v *Vertex) Abs() float64 {
	return math.Sqrt(v.X*v.X + v.Y*v.Y)
}

Interfaces are implemented implicitly

A type implements an interface by implementing its methods. There is no explicit declaration of intent, no “implements” keyword.

Implicit interfaces decouple the definition of an interface from its implementation, which could then appear in any package without prearrangement.


package main

import "fmt"

type I interface {
	M()
}

type T struct {
	S string
}

// This method means type T implements the interface I,
// but we don't need to explicitly declare that it does so.
func (t T) M() {
	fmt.Println(t.S)
}

func main() {
	var i I = T{"hello"}
	i.M()
}

Under the hood, interface values can be thought of as a tuple of a value and a concrete type:

(value, type)


package main

import (
	"fmt"
	"math"
)

type I interface {
	M()
}

type T struct {
	S string
}

func (t *T) M() {
	fmt.Println(t.S)
}

type F float64

func (f F) M() {
	fmt.Println(f)
}

func main() {
	var i I

	i = &T{"Hello"}
	describe(i)
	i.M()

	i = F(math.Pi)
	describe(i)
	i.M()
}

func describe(i I) {
	fmt.Printf("(%v, %T)\n", i, i)
}

If the concrete value inside the interface itself is nil, the method will be called with a nil receiver.


package main

import "fmt"

type I interface {
	M()
}

type T struct {
	S string
}

func (t *T) M() {
	if t == nil {
		fmt.Println("<nil>")
		return
	}
	fmt.Println(t.S)
}

func main() {
	var i I

	var t *T
	i = t
	describe(i)
	i.M()

	i = &T{"hello"}
	describe(i)
	i.M()
}

func describe(i I) {
	fmt.Printf("(%v, %T)\n", i, i)
}

The empty interface

The interface type that specifies zero methods is known as the empty interface:


interface{}

An empty interface may hold values of any type. (Every type implements at least zero methods.)

Empty interfaces are used by code that handles values of unknown type.


package main

import "fmt"

func main() {
	var i interface{}
	describe(i)

	i = 42
	describe(i)

	i = "hello"
	describe(i)
}

func describe(i interface{}) {
	fmt.Printf("(%v, %T)\n", i, i)
}

2.3 Type assertions

A type assertion provides access to an interface value’s underlying concrete value.


t := i.(T)

This statement asserts that the interface value i holds the concrete type T and assigns the underlying T value to the variable t.

If i does not hold a T, the statement will trigger a panic.

To test whether an interface value holds a specific type, a type assertion can return two values: the underlying value and a boolean value that reports whether the assertion succeeded.


t, ok := i.(T)

If i holds a T, then t will be the underlying value and ok will be true.

If not, ok will be false and t will be the zero value of type T, and no panic occurs.

Note the similarity between this syntax and that of reading from a map.


package main

import "fmt"

func main() {
	var i interface{} = "hello"

	s := i.(string)
	fmt.Println(s)

	s, ok := i.(string)
	fmt.Println(s, ok)

	f, ok := i.(float64)
	fmt.Println(f, ok)

	f = i.(float64) // panic
	fmt.Println(f)
}

2.4 Type switches

A type switch is a construct that permits several type assertions in series.

A type switch is like a regular switch statement, but the cases in a type switch specify types (not values), and those values are compared against the type of the value held by the given interface value.


switch v := i.(type) {
case T:
    // here v has type T
case S:
    // here v has type S
default:
    // no match; here v has the same type as i
}



package main

import "fmt"

func do(i interface{}) {
	switch v := i.(type) {
	case int:
		fmt.Printf("Twice %v is %v\n", v, v*2)
	case string:
		fmt.Printf("%q is %v bytes long\n", v, len(v))
	default:
		fmt.Printf("I don't know about type %T!\n", v)
	}
}

func main() {
	do(21)
	do("hello")
	do(true)
}

2.5 Stringers

One of the most ubiquitous interfaces is Stringer defined by the fmt package.


type Stringer interface {
    String() string
}

A Stringer is a type that can describe itself as a string. The fmt package (and many others) look for this interface to print values.


package main

import "fmt"

type Person struct {
	Name string
	Age  int
}

func (p Person) String() string {
	return fmt.Sprintf("%v (%v years)", p.Name, p.Age)
}

func main() {
	a := Person{"Arthur Dent", 42}
	z := Person{"Zaphod Beeblebrox", 9001}
	fmt.Println(a, z)
}

2.6 Errors

Go programs express error state with error values.

The error type is a built-in interface similar to fmt.Stringer:


type error interface {
    Error() string
}

(As with fmt.Stringer, the fmt package looks for the error interface when printing values.)

Functions often return an error value, and calling code should handle errors by testing whether the error equals nil.


i, err := strconv.Atoi("42")
if err != nil {
    fmt.Printf("couldn't convert number: %v\n", err)
    return
}
fmt.Println("Converted integer:", i)

A nil error denotes success; a non-nil error denotes failure.


package main

import (
	"fmt"
	"time"
)

type MyError struct {
	When time.Time
	What string
}

func (e *MyError) Error() string {
	return fmt.Sprintf("at %v, %s",
		e.When, e.What)
}

func run() error {
	return &MyError{
		time.Now(),
		"it didn't work",
	}
}

func main() {
	if err := run(); err != nil {
		fmt.Println(err)
	}
}

2.7 Readers

The io package specifies the io.Reader interface, which represents the read end of a stream of data.

The Go standard library contains many implementations of this interface, including files, network connections, compressors, ciphers, and others.

The io.Reader interface has a Read method:


func (T) Read(b []byte) (n int, err error)

Read populates the given byte slice with data and returns the number of bytes populated and an error value. It returns an io.EOF error when the stream ends.

The example code creates a strings.Reader and consumes its output 8 bytes at a time.


package main

import (
	"fmt"
	"io"
	"strings"
)

func main() {
	r := strings.NewReader("Hello, Reader!")

	b := make([]byte, 8)
	for {
		n, err := r.Read(b)
		fmt.Printf("n = %v err = %v b = %v\n", n, err, b)
		fmt.Printf("b[:n] = %q\n", b[:n])
		if err == io.EOF {
			break
		}
	}
}

2.8 Images

Package image defines the Image interface:


package image

type Image interface {
    ColorModel() color.Model
    Bounds() Rectangle
    At(x, y int) color.Color
}



package main

import (
	"fmt"
	"image"
)

func main() {
	m := image.NewRGBA(image.Rect(0, 0, 100, 100))
	fmt.Println(m.Bounds())
	fmt.Println(m.At(0, 0).RGBA())
}

3. Concurrency

Go provides concurrency features as part of the core language.

3.1 Goroutines

A goroutine is a lightweight thread managed by the Go runtime.

go f(x, y, z)

starts a new goroutine running


f(x, y, z)

The evaluation of f, x, y, and z happens in the current goroutine and the execution of f happens in the new goroutine.

Goroutines run in the same address space, so access to shared memory must be synchronized. The sync package provides useful primitives, although you won’t need them much in Go as there are other primitives.


package main

import (
	"fmt"
	"time"
)

func say(s string) {
	for i := 0; i < 5; i++ {
		time.Sleep(100 * time.Millisecond)
		fmt.Println(s)
	}
}

func main() {
	go say("world")
	say("hello")
}

3.2 Channels

Channels are a typed conduit through which you can send and receive values with the channel operator, <-.


ch <- v    // Send v to channel ch.
v := <-ch  // Receive from ch, and
           // assign value to v.

Like maps and slices, channels must be created before use:


ch := make(chan int)

By default, sends and receives block until the other side is ready. This allows goroutines to synchronize without explicit locks or condition variables.


package main

import "fmt"

func sum(s []int, c chan int) {
	sum := 0
	for _, v := range s {
		sum += v
	}
	c <- sum // send sum to c
}

func main() {
	s := []int{7, 2, 8, -9, 4, 0}

	c := make(chan int)
	go sum(s[:len(s)/2], c)
	go sum(s[len(s)/2:], c)
	x, y := <-c, <-c // receive from c

	fmt.Println(x, y, x+y)
}

3.3 Buffered Channels

Channels can be buffered. Provide the buffer length as the second argument to make to initialize a buffered channel:


ch := make(chan int, 100)

Sends to a buffered channel block only when the buffer is full. Receives block when the buffer is empty.


package main

import "fmt"

func main() {
	ch := make(chan int, 2)
	ch <- 1
	ch <- 2
	fmt.Println(<-ch)
	fmt.Println(<-ch)
}

3.4 Range and Close

A sender can close a channel to indicate that no more values will be sent. Receivers can test whether a channel has been closed by assigning a second parameter to the receive expression: after

v, ok := <-ch

ok is false if there are no more values to receive and the channel is closed.

The loop for i := range c receives values from the channel repeatedly until it is closed.

Note: Only the sender should close a channel, never the receiver. Sending on a closed channel will cause a panic.

Another note: Channels aren’t like files; you don’t usually need to close them. Closing is only necessary when the receiver must be told there are no more values coming, such as to terminate a range loop.


package main

import (
	"fmt"
)

func fibonacci(n int, c chan int) {
	x, y := 0, 1
	for i := 0; i < n; i++ {
		c <- x
		x, y = y, x+y
	}
	close(c)
}

func main() {
	c := make(chan int, 10)
	go fibonacci(cap(c), c)
	for i := range c {
		fmt.Println(i)
	}
}

3.5 Select

The select statement lets a goroutine wait on multiple communication operations.

A select blocks until one of its cases can run, then it executes that case. It chooses one at random if multiple are ready.


package main

import "fmt"


func fibonacci(c, quit chan int) {
	x, y := 0, 1
	for {
		select {
		case c <- x:
			x, y = y, x+y
		case <-quit:
			fmt.Println("quit")
			return
		}
	}
}

func main() {
	c := make(chan int)
	quit := make(chan int)
	go func() {
		for i := 0; i < 10; i++ {
			fmt.Println(<-c)
		}
		quit <- 0
	}()
	fibonacci(c, quit)
}

Default Selection

The default case in a select is run if no other case is ready.

Use a default case to try a send or receive without blocking:


select {
case i := <-c:
    // use i
default:
    // receiving from c would block
}



package main

import (
	"fmt"
	"time"
)

func main() {
	tick := time.Tick(100 * time.Millisecond)
	boom := time.After(500 * time.Millisecond)
	for {
		select {
		case <-tick:
			fmt.Println("tick.")
		case <-boom:
			fmt.Println("BOOM!")
			return
		default:
			fmt.Println("    .")
			time.Sleep(50 * time.Millisecond)
		}
	}
}

3.6 sync.Mutex

We’ve seen how channels are great for communication among goroutines.

But what if we don’t need communication? What if we just want to make sure only one goroutine can access a variable at a time to avoid conflicts?

This concept is called mutual exclusion, and the conventional name for the data structure that provides it is mutex.

Go’s standard library provides mutual exclusion with sync.Mutex and its two methods:


Lock
Unlock

We can define a block of code to be executed in mutual exclusion by surrounding it with a call to Lock and Unlock as shown on the Inc method.

We can also use defer to ensure the mutex will be unlocked as in the Value method.


package main

import (
	"fmt"
	"sync"
	"time"
)

// SafeCounter is safe to use concurrently.
type SafeCounter struct {
	mu sync.Mutex
	v  map[string]int
}

// Inc increments the counter for the given key.
func (c *SafeCounter) Inc(key string) {
	c.mu.Lock()
	// Lock so only one goroutine at a time can access the map c.v.
	c.v[key]++
	c.mu.Unlock()
}

// Value returns the current value of the counter for the given key.
func (c *SafeCounter) Value(key string) int {
	c.mu.Lock()
	// Lock so only one goroutine at a time can access the map c.v.
	defer c.mu.Unlock()
	return c.v[key]
}

func main() {
	c := SafeCounter{v: make(map[string]int)}
	for i := 0; i < 1000; i++ {
		go c.Inc("somekey")
	}

	time.Sleep(time.Second)
	fmt.Println(c.Value("somekey"))
}

3.7 sync.NewCond

var mu sync.Mutex
cond := sync.NewCond(&mu)

mu.Lock()
// do something that might affect the condition
cond.Broadcast()
mu.Unlock()

----

mu.Lock()
while condition == false {
    cond.Wait()
}
// now condition is true, and we have the locks
mu.Unlock()

3.8 sync.WaitGroup


package main

import "sync"

func main() {
    var wg sync.WaitGroup
    for i := 0; i < 5; i++ {
        wg.Add(1)
        go func(i int) {
            sendRPC(i)
            wg.Done() // wg.Add(-1)
        }(i)
    }
    wg.Wait()
}

func sendRPC(i int) {
    println(i)
}

Reference: