A Tour of Go

96
uangianlap
0.4 2017.04.14 22:22* 字数 6950

标签(空格分隔): 编程 Go官方文档


Using the tour

1.1 Hello, 世界

Welcome to a tour of the Go programming language.

The tour is divided into a list of modules that you can access by clicking on A Tour of Go on the top left of the page.

You can also view the table of contents at any time by clicking on the menu on the top right of the page.

Throughout the tour you will find a series of slides and exercises for you to complete.

You can navigate through them using

previous or PageUp to go to the previous page,
next or PageDown to go to the next page.
The tour is interactive. Click the Run button now (or type shift-enter) to compile and run the program on a remote server. The result is displayed below the code.

These example programs demonstrate different aspects of Go. The programs in the tour are meant to be starting points for your own experimentation.

Edit the program and run it again.

Note that when you click on Format or ctrl-enter the text in the editor is formatted using the gofmt tool. You can switch syntax highlighting on and off by clicking on the syntax button.

When you're ready to move on, click the right arrow below or type the PageDown key.

hello.go

package main

import "fmt"

func main() {
    fmt.Println("Hello, 世界")
    fmt.Println("这是一个你的故事")
}

1.2 Go local

The tour is available in other languages:

Brazilian Portuguese — Português do Brasil
Catalan — Català
[German — Deutsch]
[Spanish — Español]
[French — Français]
[Indonesia — Bahasa]
[Italian — Italiano]
[Hebrew — עִבְרִית]
[Japanese — 日本語]
[Korean — 한국어]
[Romanian — Română]
Simplified Chinese — 中文(简体)
Traditional Chinese — 中文(繁體)
[Ukrainian — Українська]
[Uzbek — Ўзбекча]
[Turkish - Türkçe]
Click the next button or type PageDown to continue.

1.3 Go offline

This tour is also available as a stand-alone program that you can use without access to the internet.

The stand-alone tour is faster, as it builds and runs the code samples on your own machine.

To run the tour locally first download and install Go then start the tour from the command line:

go tool tour
The tour program will open a web browser displaying your local version of the tour.

Or, of course, you can continue to take the tour through this web site.

1.4 The Go Playground

This tour is built atop the Go Playground, a web service that runs on golang.org's servers.

The service receives a Go program, compiles, links, and runs the program inside a sandbox, then returns the output.

There are limitations to the programs that can be run in the playground:

In the playground the time begins at 2009-11-10 23:00:00 UTC (determining the significance of this date is an exercise for the reader). This makes it easier to cache programs by giving them deterministic output.
There are also limits on execution time and on CPU and memory usage, and the program cannot access external network hosts.
The playground uses the latest stable release of Go.

Read "Inside the Go Playground" to learn more.

sandbox.go

package main

import (
    "fmt"
    "time"
)

func main() {
    fmt.Println("Welcome to the playground!")

    fmt.Println("The time is", time.Now())
}

1.5 Congratulations

You've finished the first module of the tour!
Now click on A Tour of Go to find out what else you can learn about Go, or go directly to the next lesson.

Basic

2.1 Packages,variables, and functions

2.1.1 packages

Every Go program is made up of packages.

Programs start running in package main.

This program is using the packages with import paths "fmt" and "math/rand".

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.

Note: the environment in which these programs are executed is deterministic, so each time you run the example program rand.Intn will return the same number. (To see a different number, seed the number generator; see rand.Seed.)

packages.go
package main

import (
    "fmt"
    "math/rand"
)

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

2.1.2 Imports

This code groups the imports into a parenthesized, "factored" import statement.

You can also write multiple import statements, like:

import "fmt"
import "math"
But it is good style to use the factored import statement.

imports.go
package main

import (
    "fmt"
    "math"
)

func main() {
    fmt.Printf("Now you have %g problems.", math.Sqrt(7))
}

2.1.3 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.

pizza and pi do not start with a capital letter, so they are not exported.

When importing a package, you can refer only to its exported names. Any "unexported" names are not accessible from outside the package.

Run the code. Notice the error message.

To fix the error, rename math.pi to math.Pi and try it again.

exported-names.go
package main

import (
    "fmt"
    "math"
)

func main() {
    fmt.Println(math.pi)
}

2.1.4 Functions

A function can take zero or more arguments.

In this example, add takes two parameters of type int.

Notice that the type comes after the variable name.

(For more about why types look the way they do, see the article on Go's declaration syntax.)

functions.go
package main

import "fmt"

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

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

2.1.5 Functions continued

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

functions-continued.go
package main

import "fmt"

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

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

2.1.6 Multiple results

A function can return any number of results.

The swap function returns two strings.

multiple-results.go
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)
}

2.1.7 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.

A return statement without arguments returns the named return values. This is known as a "naked" return.

Naked return statements should be used only in short functions, as with the example shown here. They can harm readability in longer functions.

named-results.go
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))
}

2.1.8 Variables

The varstatement 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.

variables.go
package main

import "fmt"

var c, python, java bool

func main() {
    var i int
    fmt.Println(i, c, python, java)
}

2.1.9 Variables with initializers

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.

variables-with-initializers.go
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)
}

2.1.10 Short variable declarations

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.

short-variable-declarle-daclarations.go
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)
}

2.1.11 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

The example shows variables of several types, and also that variable declarations may be "factored" into blocks, as with import statements.

The int, uint, and uintptr types are usually 32 bits wide on 32-bit systems and 64 bits wide on 64-bit systems. When you need an integer value you should use int unless you have a specific reason to use a sized or unsigned integer type.

basic-types.go
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)
}

2.1.12 Zero values

Variables declared without an explicit initial value are given their zero value.
The zero value is:

    0 for numeric types,
    false for the boolean type, and
    "" (the empty string) for strings.

zero.go

package main

import "fmt"

func main() {
    var i int
    var f float64
    var b bool
    var s string
    fmt.Printf("%v %v %v %q\n", i, f, b, s)
}

2.1.13 Type conversions

The expression T(v) converts the value v to the type T.

Some numeric conversions:

var i int = 42
var f float64 = float64(i)
var u uint = uint(f)

Or, put more simply:

i := 42
f := float64(i)
u := uint(f)

Unlike in C, in Go assignment between items of different type requires an explicit conversion. Try removing the float64 or uint conversions in the example and see what happens.

type-conversions.go

package main

import (
    "fmt"
    "math"
)

func main() {
    var x, y int = 3, 4
    var f float64 = math.Sqrt(float64(x*x + y*y))
    var z uint = uint(f)
    fmt.Println(x, y, z)
}

2.1.14 Type inference

When declaring a variable without specifying an explicit type (either by using the := syntax or var = expression syntax), the variable's type is inferred from the value on the right hand side.

When the right hand side of the declaration is typed, the new variable is of that same type:

var i int
j := i // j is an int

But when the right hand side contains an untyped numeric constant, the new variable may be an int, float64, or complex128 depending on the precision of the constant:

i := 42 // int
f := 3.142 // float64
g := 0.867 + 0.5i // complex128

Try changing the initial value of v in the example code and observe how its type is affected.

type-inference.go

package main

import "fmt"

func main() {
    v := 42 // change me!
    fmt.Printf("v is of type %T\n", v)
}

2.1.15 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.

constants.go

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)
}

2.1.16 Numeric Constants

Numeric constants are high-precision values.

An untyped constant takes the type needed by its context.

Try printing needInt(Big) too.

(An int can store at maximum a 64-bit integer, and sometimes less.)

numeric-constants.go

package main

import "fmt"

const (
    // Create a huge number by shifting a 1 bit left 100 places.
    // In other words, the binary number that is 1 followed by 100 zeroes.
    Big = 1 << 100
    // Shift it right again 99 places, so we end up with 1<<1, or 2.
    Small = Big >> 99
)

func needInt(x int) int { return x*10 + 1 }
func needFloat(x float64) float64 {
    return x * 0.1
}

func main() {
    fmt.Println(needInt(Small))
    fmt.Println(needFloat(Small))
    fmt.Println(needFloat(Big))
}

2.1.17 Congratulations!

You finished this lesson!

You can go back to the list of modules to find what to learn next, or continue with the next lesson.

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

2.2.1 For

Go has only one looping construct, the for loop.

The basic for loop has three components separated by semicolons:

the init statement: executed before the first iteration
the condition expression: evaluated before every iteration
the post statement: executed at the end of every iteration
The init statement will often be a short variable declaration, and the variables declared there are visible only in the scope of the for statement.

The loop will stop iterating once the boolean condition evaluates to false.

Note: Unlike other languages like C, Java, or Javascript there are no parentheses surrounding the three components of the for statement and the braces { } are always required.

for.go

package main

import "fmt"

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

2.2.2 For continued

The init and post statement are optional.

for-continued.go

package main

import "fmt"

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

2.2.3 For is Go's "while"

At that point you can drop the semicolons: C's while is spelled for in Go.

for-is-gos-while.go

package main

import "fmt"

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

2.2.4 Forever

If you omit the loop condition it loops forever, so an infinite loop is compactly expressed.

forever.go

package main

func main() {
    for {
    }
}

2.2.5 If

Go's if statements are like its for loops; the expression need not be surrounded by parentheses ( ) but the braces { } are required.

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))
}

2.2.6 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.

(Try using v in the last return statement.)

if-with-a-short-statement.go

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),
    )
}

2.2.7 If and else

Variables declared inside an if short statement are also available inside any of the else blocks.

(Both calls to pow are executed and return before the call to fmt.Println in main begins.)

if-and-else.go

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),
    )
}

2.2.8 Exercise:Loops and Functions

As a simple way to play with functions and loops, implement the square root function using Newton's method.

In this case, Newton's method is to approximate Sqrt(x) by picking a starting point z and then repeating:

To begin with, just repeat that calculation 10 times and see how close you get to the answer for various values (1, 2, 3, ...).

Next, change the loop condition to stop once the value has stopped changing (or only changes by a very small delta). See if that's more or fewer iterations. How close are you to the math.Sqrt?

Hint: to declare and initialize a floating point value, give it floating point syntax or use a conversion:

z := float64(1)
z := 1.0

2.2.9 Switch

You probably knew what switch was going to look like.

A case body breaks automatically, unless it ends with a fallthrough statement.

switch.go

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.", os)
    }
}

2.2.10 Switch evaluation order

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.)

Note: Time in the Go playground always appears to start at 2009-11-10 23:00:00 UTC, a value whose significance is left as an exercise for the reader.

switch-evaluation-order.go

package main

import (
    "fmt"
    "time"
)

func main() {
    fmt.Println("When's Saturday?")
    today := time.Now().Weekday()
    switch time.Saturday {
    case today + 0:
        fmt.Println("Today.")
    case today + 1:
        fmt.Println("Tomorrow.")
    case today + 2:
        fmt.Println("In two days.")
    default:
        fmt.Println("Too far away.")
    }
}

2.2.11 Switch with no condition

Switch without a condition is the same as switch true.

This construct can be a clean way to write long if-then-else chains.

switch-with-no-condition.go

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.")
    }
}

2.2.12 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.

defer.go

package main

import "fmt"

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

    fmt.Println("hello")
}

2.2.13 Stacking defers

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

To learn more about defer statements read this blog post.

defer-multi.go

package main

import "fmt"

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

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

    fmt.Println("done")
}

2.2.14 Congratulations!

You finished this lesson!

You can go back to the list of modules to find what to learn next, or continue with the next lesson.

2.3 More types: structs, slices, and maps

2.3.1 Pointers

Go has pointers. A pointer holds the memory address of a value.

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

The * operator denotes the pointer's underlying value.

fmt.Println(*p) // read i through the pointer p
*p = 21 // set i through the pointer p

This is known as "dereferencing" or "indirecting".

Unlike C, Go has no pointer arithmetic.

pointers.go

ppackage 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
}

2.3.2 Structs

A struct is a collection of fields.

(And a type declaration does what you'd expect.)

structs.go

package main

import "fmt"

type Vertex struct {
    X int
    Y int
}

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

2.3.3 Struct Fields

Struct fields are accessed using a dot.

struct-fields.go

package main

import "fmt"

type Vertex struct {
    X int
    Y int
}

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

2.3.4 Pointers to structs

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.

struct-pointers.go

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)
}

2.3.5 Struct Literals

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.

struct-literals.go

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)
}

2.3.6 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.

array.go

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)
}

2.3.7 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 type []T is a slice with elements of type T.

This expression creates a slice of the first five elements of the array a:

a[0:5]

slices.go

package main

import "fmt"

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

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

2.3.8 Slices are like references to arrays

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

Changing the elements of a slice modifies the corresponding elements of its underlying array.

Other slices that share the same underlying array will see those changes.

slices-pointers.go

package main

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)
}

2.3.9 Slice literals

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}

slice-literals.go

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)
}

2.3.10 Slice defaults

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[:]

slice-bounds.go

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)
}

2.3.11 Slice length and capacity

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).

You can extend a slice's length by re-slicing it, provided it has sufficient capacity. Try changing one of the slice operations in the example program to extend it beyond its capacity and see what happens.

slice-len-cap.go

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)
}

2.3.12 Nil slices

The zero value of a slice is nil.

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

nil-slices.go

package main

import "fmt"

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

2.3.13 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

making-slices.go

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)
}

2.3.14 Slices of slices

Slices can contain any type, including other slices.

slices-of-slice.go

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], " "))
    }
}

2.3.15 Appending to slice

It is common to append new elements to a slice, and so Go provides a built-in append function. The documentation of the built-in package describes append.

func append(s []T, vs ...T) []T

The first parameter s of append is a slice of type T, and the rest are T values to append to the slice.

The resulting value of append is a slice containing all the elements of the original slice plus the provided values.

If the backing array of s is too small to fit all the given values a bigger array will be allocated. The returned slice will point to the newly allocated array.

(To learn more about slices, read the Slices: usage and internals article.)

append.go

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)
}

2.3.16 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.

range.go

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)
    }
}

2.3.17 Range continued

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

If you only want the index, drop the ", value" entirely.

range-continued.go

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)
    }
}

2.3.18 Exercise: Slices

Implement Pic. It should return a slice of length dy, each element of which is a slice of dx 8-bit unsigned integers. When you run the program, it will display your picture, interpreting the integers as grayscale (well, bluescale) values.

The choice of image is up to you. Interesting functions include (x+y)/2, x*y, and x^y.

(You need to use a loop to allocate each []uint8 inside the [][]uint8.)

(Use uint8(intValue) to convert between types.)

exercise-slices.go

package main

import "golang.org/x/tour/pic"

func Pic(dx, dy int) [][]uint8 {
}

func main() {
    pic.Show(Pic)
}

2.3.19 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.

maps.go

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"])
}

2.3.20 Map literals

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

map-literals.go

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)
}

2.3.21 Map literals continued

If the top-level type is just a type name, you can omit it from the elements of the literal.

map-literals-continued.go

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)
}

2.3.22 Mutating Maps

Insert or update an element in map m:

m[key] = elem

Retrieve an element:

elem = m[key]

Delete an element:

delete(m, key)

Test that a key is present with a two-value assignment:

elem, ok = m[key]

If key is in m, ok is true. If not, ok is false.

If key is not in the map, then elem is the zero value for the map's element type.

Note: if elem or ok have not yet been declared you could use a short declaration form:

elem, ok := m[key]

mutating-maps.go

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)
}

2.3.23 Exercise: Maps

Implement WordCount. It should return a map of the counts of each “word” in the string s. The wc.Test function runs a test suite against the provided function and prints success or failure.

You might find strings.Fields helpful.

exercise-maps.go

package main

import (
    "golang.org/x/tour/wc"
)

func WordCount(s string) map[string]int {
    return map[string]int{"x": 1}
}

func main() {
    wc.Test(WordCount)
}

2.3.24 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.

function-values.go

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))
}

2.3.25 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.

function-closures.go

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),
        )
    }
}

2.3.26 Exercise: Fibonacci closure

Let's have some fun with functions.

Implement a fibonacci function that returns a function (a closure) that returns successive fibonacci numbers (0, 1, 1, 2, 3, 5, ...).

exercise-fibonacci-closure.go

package main

import "fmt"

// fibonacci is a function that returns
// a function that returns an int.
func fibonacci() func() int {
}

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

2.3.27 Congratulations!

You finished this lesson!

You can go back to the list of modules to find what to learn next, or continue with the next lesson.

Methods and interfaces

3.1 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.

In this example, the Abs method has a receiver of type Vertex named v.

methods.go

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())
}

3.2 Methods are functions

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

Here's Abs written as a regular function with no change in functionality.

methods-funcs.go

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))
}

3.3 Methods continued

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

In this example we see a numeric type MyFloat with an Abs method.

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).

methods-continued.go

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())
}

3.4 Pointer receivers

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.)

For example, the Scale method here is defined on *Vertex.

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.

Try removing the * from the declaration of the Scale function on line 16 and observe how the program's behavior changes.

With a value receiver, the Scale method operates on a copy of the original Vertex value. (This is the same behavior as for any other function argument.) The Scale method must have a pointer receiver to change the Vertex value declared in the main function.

methods-pointers.go

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())
}

3.5 Pointers and functions

Here we see the Abs and Scale methods rewritten as functions.

Again, try removing the * from line 16. Can you see why the behavior changes? What else did you need to change for the example to compile?

(If you're not sure, continue to the next page.)

methods-pointers-explained.go

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

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

3.6 Methods and pointer indirection

Comparing the previous two programs, you might notice that functions with a pointer argument must take a pointer:

var v Vertex
ScaleFunc(v) // Compile error!
ScaleFunc(&v) // OK

while 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.

indirection.go

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)
}

3.7 Methods and pointer indirection (2)

The equivalent thing happens in the reverse direction.

Functions that take a value argument must take a value of that specific type:

var v Vertex
fmt.Println(AbsFunc(v)) // OK
fmt.Println(AbsFunc(&v)) // Compile error!

while 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().

indirection-values.go

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))
}

3.8 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, for example.

In this example, both Scale and Abs are with receiver type *Vertex, even though the Abs method needn't modify its receiver.

In general, all methods on a given type should have either value or pointer receivers, but not a mixture of both. (We'll see why over the next few pages.)

methods-with-pointer-receivers.go

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())
}

3.9 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).

interfaces.go

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)
}

3.10 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.

interfaces-are-satisfied-implicitly.go

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()
}

3.11 Interface values

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

(value, type)

An interface value holds a value of a specific underlying concrete type.

Calling a method on an interface value executes the method of the same name on its underlying type.

interface-values.go

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)
}

3.12 Interface values with nil underlying values

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

In some languages this would trigger a null pointer exception, but in Go it is common to write methods that gracefully handle being called with a nil receiver (as with the method M in this example.)

Note that an interface value that holds a nil concrete value is itself non-nil.

interface-values-with-nil.go

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)
}

3.13 Nil interface values

A nil interface value holds neither value nor concrete type.

Calling a method on a nil interface is a run-time error because there is no type inside the interface tuple to indicate which concrete method to call.

nil-interface-values.go

package main

import "fmt"

type I interface {
    M()
}

func main() {
    var i I
    describe(i)
    i.M()
}

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

3.14 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. For example, fmt.Print takes any number of arguments of type interface{}.

empty-interface.go

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)
}

3.15 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.

type-assertions.go

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)
}

3.16 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
}

The declaration in a type switch has the same syntax as a type assertion i.(T), but the specific type T is replaced with the keyword type.

This switch statement tests whether the interface value i holds a value of type T or S. In each of the T and S cases, the variable v will be of type T or S respectively and hold the value held by i. In the default case (where there is no match), the variable v is of the same interface type and value as i.

type-switches.go

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)
}

3.17 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.

stringer.go

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)
}

3.18 Exercise: Stringers

Make the IPAddr type implement fmt.Stringer to print the address as a dotted quad.

For instance, IPAddr{1, 2, 3, 4} should print as "1.2.3.4".

exercise-stringer.go

package main

import "fmt"

type IPAddr [4]byte

// TODO: Add a "String() string" method to IPAddr.

func main() {
    hosts := map[string]IPAddr{
        "loopback":  {127, 0, 0, 1},
        "googleDNS": {8, 8, 8, 8},
    }
    for name, ip := range hosts {
        fmt.Printf("%v: %v\n", name, ip)
    }
}

3.19 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.

errors.go

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)
    }
}

3.20 Exercise: Errors

Copy your Sqrt function from the earlier exercise and modify it to return an error value.

Sqrt should return a non-nil error value when given a negative number, as it doesn't support complex numbers.

Create a new type

type ErrNegativeSqrt float64

and make it an error by giving it a

func (e ErrNegativeSqrt) Error() string

method such that ErrNegativeSqrt(-2).Error() returns "cannot Sqrt negative number: -2".

Note: a call to fmt.Sprint(e) inside the Error method will send the program into an infinite loop. You can avoid this by converting e first: fmt.Sprint(float64(e)). Why?

Change your Sqrt function to return an ErrNegativeSqrt value when given a negative number.

exercise-errors.go

package main

import (
    "fmt"
)

func Sqrt(x float64) (float64, error) {
    return 0, nil
}

func main() {
    fmt.Println(Sqrt(2))
    fmt.Println(Sqrt(-2))
}

3.21 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 these interfaces, 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.

reader.go

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
        }
    }
}

3.22 Exercise: Readers

Implement a Reader type that emits an infinite stream of the ASCII character 'A'.

exercise-reader.go

package main

import "golang.org/x/tour/reader"

type MyReader struct{}

// TODO: Add a Read([]byte) (int, error) method to MyReader.

func main() {
    reader.Validate(MyReader{})
}

3.23 Exercise: rot13Reader

A common pattern is an io.Reader that wraps another io.Reader, modifying the stream in some way.

For example, the gzip.NewReader function takes an io.Reader (a stream of compressed data) and returns a *gzip.Reader that also implements io.Reader (a stream of the decompressed data).

Implement a rot13Reader that implements io.Reader and reads from an io.Reader, modifying the stream by applying the rot13 substitution cipher to all alphabetical characters.

The rot13Reader type is provided for you. Make it an io.Reader by implementing its Read method.

exercise-rot-reader.go

package main

import (
    "io"
    "os"
    "strings"
)

type rot13Reader struct {
    r io.Reader
}

func main() {
    s := strings.NewReader("Lbh penpxrq gur pbqr!")
    r := rot13Reader{s}
    io.Copy(os.Stdout, &r)
}

3.24 Images

Package image defines the Image interface:

package image

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

Note: the Rectangle return value of the Bounds method is actually an image.Rectangle, as the declaration is inside package image.

(See the documentation for all the details.)

The color.Color and color.Model types are also interfaces, but we'll ignore that by using the predefined implementations color.RGBA and color.RGBAModel. These interfaces and types are specified by the image/color package

3.25 Exercise: Images

Remember the picture generator you wrote earlier? Let's write another one, but this time it will return an implementation of image.Image instead of a slice of data.

Define your own Image type, implement the necessary methods, and call pic.ShowImage.

Bounds should return a image.Rectangle, like image.Rect(0, 0, w, h).

ColorModel should return color.RGBAModel.

At should return a color; the value v in the last picture generator corresponds to color.RGBA{v, v, 255, 255} in this one.

exercise-images.go

package main

import "golang.org/x/tour/pic"

type Image struct{}

func main() {
    m := Image{}
    pic.ShowImage(m)
}

3.26 Congratulations!

You finished this lesson!

You can go back to the list of modules to find what to learn next, or continue with the next lesson.

4. Concurrency

Go provides concurrency features as part of the core language.

This module goes over goroutines and channels, and how they are used to implement different concurrency patterns.

4.1 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. (See the next slide.)

goroutines.go

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")
}

4.2 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.

(The data flows in the direction of the arrow.)

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.

The example code sums the numbers in a slice, distributing the work between two goroutines. Once both goroutines have completed their computation, it calculates the final result.

channels.go

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)
}

4.3 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.

Modify the example to overfill the buffer and see what happens.

buffered-channels.go

package main

import "fmt"

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

4.4 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.

range-and-close.go

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)
    }
}

4.5 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.

select.go

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)
}

4.6 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
}

default-selection.go

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)
        }
    }
}

4.7 Exercise: Equivalent Binary Trees

There can be many different binary trees with the same sequence of values stored at the leaves. For example, here are two binary trees storing the sequence 1, 1, 2, 3, 5, 8, 13.

A function to check whether two binary trees store the same sequence is quite complex in most languages. We'll use Go's concurrency and channels to write a simple solution.
This example uses the tree
package, which defines the type:

type Tree struct { 
    Left *Tree 
    Value int 
    Right *Tree
}

Continue description on next page.

4.8 Exercise: Equivalent Binary Trees

  1. Implement the Walk function.

  2. Test the Walk function.

The function tree.New(k) constructs a randomly-structured binary tree holding the values k, 2k, 3k, ..., 10k.

Create a new channel ch and kick off the walker:

go Walk(tree.New(1), ch)

Then read and print 10 values from the channel. It should be the numbers 1, 2, 3, ..., 10.

  1. Implement the Same function using Walk to determine whether t1 and t2 store the same values.

  2. Test the Same function.

Same(tree.New(1), tree.New(1)) should return true, and Same(tree.New(1), tree.New(2)) should return false.

The documentation for Tree can be found here.

exeercise-equivalent-binary-trees.go

package main

import "golang.org/x/tour/tree"

// Walk walks the tree t sending all values
// from the tree to the channel ch.
func Walk(t *tree.Tree, ch chan int)

// Same determines whether the trees
// t1 and t2 contain the same values.
func Same(t1, t2 *tree.Tree) bool

func main() {
}

4.9 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.

mutex-counter.go

package main

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

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

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

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

4.10 Exercise: Web Crawler

In this exercise you'll use Go's concurrency features to parallelize a web crawler.

Modify the Crawl function to fetch URLs in parallel without fetching the same URL twice.

Hint: you can keep a cache of the URLs that have been fetched on a map, but maps alone are not safe for concurrent use!

exercise-web-crawler.go

package main

import (
    "fmt"
)

type Fetcher interface {
    // Fetch returns the body of URL and
    // a slice of URLs found on that page.
    Fetch(url string) (body string, urls []string, err error)
}

// Crawl uses fetcher to recursively crawl
// pages starting with url, to a maximum of depth.
func Crawl(url string, depth int, fetcher Fetcher) {
    // TODO: Fetch URLs in parallel.
    // TODO: Don't fetch the same URL twice.
    // This implementation doesn't do either:
    if depth <= 0 {
        return
    }
    body, urls, err := fetcher.Fetch(url)
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Printf("found: %s %q\n", url, body)
    for _, u := range urls {
        Crawl(u, depth-1, fetcher)
    }
    return
}

func main() {
    Crawl("http://golang.org/", 4, fetcher)
}

// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult

type fakeResult struct {
    body string
    urls []string
}

func (f fakeFetcher) Fetch(url string) (string, []string, error) {
    if res, ok := f[url]; ok {
        return res.body, res.urls, nil
    }
    return "", nil, fmt.Errorf("not found: %s", url)
}

// fetcher is a populated fakeFetcher.
var fetcher = fakeFetcher{
    "http://golang.org/": &fakeResult{
        "The Go Programming Language",
        []string{
            "http://golang.org/pkg/",
            "http://golang.org/cmd/",
        },
    },
    "http://golang.org/pkg/": &fakeResult{
        "Packages",
        []string{
            "http://golang.org/",
            "http://golang.org/cmd/",
            "http://golang.org/pkg/fmt/",
            "http://golang.org/pkg/os/",
        },
    },
    "http://golang.org/pkg/fmt/": &fakeResult{
        "Package fmt",
        []string{
            "http://golang.org/",
            "http://golang.org/pkg/",
        },
    },
    "http://golang.org/pkg/os/": &fakeResult{
        "Package os",
        []string{
            "http://golang.org/",
            "http://golang.org/pkg/",
        },
    },
}

4.11 Where to Go from here...

You can get started by installing Go.

Once you have Go installed, the Go Documentation is a great place to continue. It contains references, tutorials, videos, and more.

To learn how to organize and work with Go code, watch this screencast or read How to Write Go Code.

If you need help with the standard library, see the package reference. For help with the language itself, you might be surprised to find the Language Spec is quite readable.

To further explore Go's concurrency model, watch Go Concurrency Patterns (slides) and Advanced Go Concurrency Patterns (slides) and read the Share Memory by Communicating codewalk.

To get started writing web applications, watch A simple programming environment (slides) and read the Writing Web Applications tutorial.

The First Class Functions in Go codewalk gives an interesting perspective on Go's function types.

The Go Blog has a large archive of informative Go articles.

Visit golang.org for more.

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