Hello, 世界

Welcome to a tour of the Go programming language.

The tour is divided into three sections: basic concepts, methods and interfaces, and concurrency.

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

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.

When you're ready to move on, click the right arrow down below or type the PageDown key. You can also navigate using the menu under the "Go" flag at the top of the page.

package main

import "fmt"

func main() {
    fmt.Println("Hello, 世界")
}

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:

  • The playground can use most of the standard library, with some exceptions; notably absent are network and file system access. Therefore, the only communication a playground program has to the outside world is by writing to standard output and standard error.
  • 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 is restricted to running in a single thread (but may use many goroutines).

The playground uses the latest stable release of Go.

package main

import (
    "fmt"
    "net"
    "os"
    "time"
)

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

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

    fmt.Println("And if you try to open a file:")
    fmt.Println(os.Open("filename"))

    fmt.Println("Or access the network:")
    fmt.Println(net.Dial("tcp", "google.com"))
}

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 rand.Intn will always return the same number.

(To see a different number, seed the number generator; see rand.Seed.)

package main

import (
    "fmt"
    "math/rand"
)

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

Imports

This code groups the imports into a parenthesized, "factored" import statement. You can also write multiple import statements, like:

import "fmt"
import "math"
package main

import (
    "fmt"
    "math"
)

func main() {
    fmt.Printf("Now you have %g problems.",
        math.Nextafter(2, 3))
}

Exported names

After importing a package, you can refer to the names it exports.

In Go, a name is exported if it begins with a capital letter.

Foo is an exported name, as is FOO. The name foo is not exported.

Run the code. Then rename math.pi to math.Pi and try it again.

package main

import (
    "fmt"
    "math"
)

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

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.)

package main

import "fmt"

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

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

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
package main

import "fmt"

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

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

Multiple results

A function can return any number of results.

This function returns two strings.

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)
}

Named results

Functions take parameters. In Go, functions can return multiple "result parameters", not just a single value. They can be named and act just like variables.

If the result parameters are named, a return statement without arguments returns the current values of the results.

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))
}

Variables

The var statement declares a list of variables; as in function argument lists, the type is last.

package main

import "fmt"

var i int
var c, python, java bool

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

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.

package main

import "fmt"

var i, j int = 1, 2
var c, python, java = true, false, "no!"

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

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 construct begins with a keyword (var, func, and so on) and 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)
}

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() {
    const f = "%T(%v)\n"
    fmt.Printf(f, ToBe, ToBe)
    fmt.Printf(f, MaxInt, MaxInt)
    fmt.Printf(f, z, z)
}

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 int conversions in the example and see what happens.

package main

import (
    "fmt"
    "math"
)

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

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)
}

Numeric Constants

Numeric constants are high-precision values.

An untyped constant takes the type needed by its context.

Try printing needInt(Big) too.

package main

import "fmt"

const (
    Big   = 1 << 100
    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))
}

For

Go has only one looping construct, the for loop.

The basic for loop looks as it does in C or Java, except that the ( ) are gone (they are not even optional) and the { } are required.

package main

import "fmt"

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

For continued

As in C or Java, you can leave the pre and post statements empty.

package main

import "fmt"

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

For is Go's "while"

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

package main

import "fmt"

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

Forever

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

package main

func main() {
    for {
    }
}

If

The if statement looks as it does in C or Java, except that the ( ) are gone and the { } are required.

(Sound familiar?)

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.

(Try using v in the last return statement.)

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),
    )
}

If and else

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

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),
    )
}

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
package main

import (
    "fmt"
)

func Sqrt(x float64) float64 {
}

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

Structs

A struct is a collection of fields.

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

package main

import "fmt"

type Vertex struct {
    X int
    Y int
}

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

Struct Fields

Struct fields are accessed using a dot.

package main

import "fmt"

type Vertex struct {
    X int
    Y int
}

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

Pointers

Go has pointers, but no pointer arithmetic.

Struct fields can be accessed through a struct pointer. The indirection through the pointer is transparent.

package main

import "fmt"

type Vertex struct {
    X int
    Y int
}

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

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 & constructs a pointer to a newly allocated struct.

package main

import "fmt"

type Vertex struct {
    X, Y int
}

var (
    p = Vertex{1, 2}  // has type Vertex
    q = &Vertex{1, 2} // has type *Vertex
    r = Vertex{X: 1}  // Y:0 is implicit
    s = Vertex{}      // X:0 and Y:0
)

func main() {
    fmt.Println(p, q, r, s)
}

The new function

The expression new(T) allocates a zeroed T value and returns a pointer to it.

var t *T = new(T)

or

t := new(T)
package main

import "fmt"

type Vertex struct {
    X, Y int
}

func main() {
    v := new(Vertex)
    fmt.Println(v)
    v.X, v.Y = 11, 9
    fmt.Println(v)
}

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)
}

Slices

A slice points to an array of values and also includes a length.

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

package main

import "fmt"

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

    for i := 0; i < len(p); i++ {
        fmt.Printf("p[%d] == %d\n", i, p[i])
    }
}

Slicing slices

Slices can be re-sliced, creating a new slice value that points to the same array.

The expression

s[lo:hi]

evaluates to a slice of the elements from lo through hi-1, inclusive. Thus

s[lo:lo]

is empty and

s[lo:lo+1]

has one element.

package main

import "fmt"

func main() {
    p := []int{2, 3, 5, 7, 11, 13}
    fmt.Println("p ==", p)
    fmt.Println("p[1:4] ==", p[1:4])

    // missing low index implies 0
    fmt.Println("p[:3] ==", p[:3])

    // missing high index implies len(s)
    fmt.Println("p[4:] ==", p[4:])
}

Making slices

Slices are created with the make function. It works by allocating a zeroed array and returning 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)
}

Nil slices

The zero value of a slice is nil.

A nil slice has a length and capacity of 0.

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

package main

import "fmt"

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

Range

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

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)
    }
}

Range continued

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

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

package main

import "fmt"

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

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, (x+y)/2, and x*y.

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

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

package main

import "code.google.com/p/go-tour/pic"

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

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

Maps

A map maps keys to values.

Maps must be created with make (not new) before use; the nil map is empty and cannot be assigned to.

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

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)
}

Map literals continued

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

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

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 and elem is the zero value for the map's element type.

Similarly, when reading from a map if the key is not present the result is the zero value for the map's element type.

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)
}

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.

package main

import (
    "code.google.com/p/go-tour/wc"
)

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

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

Function values

Functions are values too.

package main

import (
    "fmt"
    "math"
)

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

    fmt.Println(hypot(3, 4))
}

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),
        )
    }
}

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.

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())
    }
}

Switch

You probably knew what switch was going to look like.

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

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)
    }
}

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.

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.")
    }
}

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.

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.")
    }
}

Advanced Exercise: Complex cube roots

Let's explore Go's built-in support for complex numbers via the complex64 and complex128 types. For cube roots, Newton's method amounts to repeating:

Find the cube root of 2, just to make sure the algorithm works. There is a Pow function in the math/cmplx package.

package main

import "fmt"

func Cbrt(x complex128) complex128 {
}

func main() {
    fmt.Println(Cbrt(2))
}

Methods and Interfaces

The next group of slides covers methods and interfaces, the constructs that define objects and their behavior.

Methods

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

The method 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())
}

Methods continued

In fact, you can define a method on any type you define in your package, not just structs.

You cannot define a method on a type from another package, or on a basic type.

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())
}

Methods with pointer receivers

Methods can be associated with a named type or a pointer to a named type.

We just saw two Abs methods. One on the *Vertex pointer type and the other on the MyFloat value type.

There are two reasons to use a pointer receiver. First, to avoid copying the value on each method call (more efficient if the value type is a large struct). Second, so that the method can modify the value that its receiver points to.

Try changing the declarations of the Abs and Scale methods to use Vertex as the receiver, instead of *Vertex.

The Scale method has no effect when v is a Vertex. Scale mutates v. When v is a value (non-pointer) type, the method sees a copy of the Vertex and cannot mutate the original value.

Abs works either way. It only reads v. It doesn't matter whether it is reading the original value (through a pointer) or a copy of that value.

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}
    v.Scale(5)
    fmt.Println(v, v.Abs())
}

Interfaces

An interface type is defined by a set of methods.

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

Note: The code on the left fails to compile.

Vertex doesn't satisfy Abser because the Abs method is defined only on *Vertex, not Vertex.

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 satisfied implicitly

A type implements an interface by implementing the methods.

There is no explicit declaration of intent.

Implicit interfaces decouple implementation packages from the packages that define the interfaces: neither depends on the other.

It also encourages the definition of precise interfaces, because you don't have to find every implementation and tag it with the new interface name.

Package io defines Reader and Writer; you don't have to.

package main

import (
    "fmt"
    "os"
)

type Reader interface {
    Read(b []byte) (n int, err error)
}

type Writer interface {
    Write(b []byte) (n int, err error)
}

type ReadWriter interface {
    Reader
    Writer
}

func main() {
    var w Writer

    // os.Stdout implements Writer
    w = os.Stdout

    fmt.Fprintf(w, "hello, writer\n")
}

Errors

An error is anything that can describe itself as an error string. The idea is captured by the predefined, built-in interface type, error, with its single method, Error, returning a string:

type error interface {
    Error() string
}

The fmt package's various print routines automatically know to call the method when asked to print an error.

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)
    }
}

Exercise: Errors

Copy your Sqrt function from the earlier exercises 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.Print(e) inside the Error method will send the program into an infinite loop. You can avoid this by converting e first: fmt.Print(float64(e)). Why?

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

package main

import (
    "fmt"
)

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

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

Web servers

Package http serves HTTP requests using any value that implements http.Handler:

package http

type Handler interface {
    ServeHTTP(w ResponseWriter, r *Request)
}

In this example, the type Hello implements http.Handler.

Visit http://localhost:4000/ to see the greeting.

Note: This example won't run through the web-based tour user interface. To try writing web servers you may want to Install Go.

package main

import (
    "fmt"
    "net/http"
)

type Hello struct{}

func (h Hello) ServeHTTP(
    w http.ResponseWriter,
    r *http.Request) {
    fmt.Fprint(w, "Hello!")
}

func main() {
    var h Hello
    http.ListenAndServe("localhost:4000", h)
}

Exercise: HTTP Handlers

Implement the following types and define ServeHTTP methods on them. Register them to handle specific paths in your web server.

type String string

type Struct struct {
    Greeting string
    Punct    string
    Who      string
}

For example, you should be able to register handlers using:

http.Handle("/string", String("I'm a frayed knot."))
http.Handle("/struct", &Struct{"Hello", ":", "Gophers!"})
package main

import (
    "net/http"
)

func main() {
    // your http.Handle calls here
    http.ListenAndServe("localhost:4000", nil)
}

Images

Package image defines the Image interface:

package image

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

(See the documentation for all the details.)

Also, color.Color and color.Model are 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

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())
}

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.

package main

import (
    "code.google.com/p/go-tour/pic"
    "image"
)

type Image struct{}

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

Exercise: Rot13 Reader

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 gzipped 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.

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)
}

Concurrency

The next section covers Go's concurrency primitives.

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.)

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")
}

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.

package main

import "fmt"

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

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

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

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

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.

package main

import "fmt"

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

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)
    }
}

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)
        }
    }
}

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
}

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.

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

4. 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.

package main

import "code.google.com/p/go-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() {
}

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.

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/",
        },
    },
}

Where to Go from here...

You can get started by installing Go or downloading the Go App Engine SDK.

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.