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

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

ch := make(chan int)
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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.

Basic Example

package main

import "fmt"

func sum(s []int, c chan int) {
    fmt.Println("%v", s)
    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)
}
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Prints:

%v [-9 4 0]
%v [7 2 8]
-5 17 12
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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)
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Sends to a buffered channel block only when the buffer is full. Receives block when the buffer is empty.

If you overflow the buffer, you'll be hit with a deadlock error.

package main

import "fmt"

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

0
1
1
2
3
5
8
13
21
34
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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)
}
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After looping through in the IIFE go func, it will send a 0 to the quit channel and select will handle by printing "quit" and returning from the infinite for loop.

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
}
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Example:

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)
        }
    }
}
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Example: Equivalent Binary Trees

package main

import (
  "golang.org/x/tour/tree"
  "fmt"
)

// Walk walks the tree t sending all values
// from the tree to the channel ch.
func Walk(t *tree.Tree, ch chan int) {
    defer close(ch) // <- closes the channel when this function returns
  var walk func(t *tree.Tree)
  walk = func(t *tree.Tree) {
      if t == nil {
          return
      }
      walk(t.Left)
      ch <- t.Value
      walk(t.Right)
  }
  walk(t)
}

// Same determines whether the trees
// t1 and t2 contain the same values.
func Same(t1, t2 *tree.Tree) bool {
    done := make (chan bool)
    defer close(done)

    ch1 := make(chan int)
    ch2 := make(chan int)
    go Walk(t1, ch1)
    go Walk(t2, ch2)

    go func() {
        for i := range ch1 {
            j := <-ch2
            fmt.Println("i: %v", i)
            fmt.Println("j: %v", j)

            if i != j {
                done <- false
            }
        }
        done <- true
    }()
    return <-done
}

func main() {
    ch := make(chan int)
    go Walk(tree.New(1), ch)

    for i := 1; i <= 10; i++ {
        fmt.Println(<-ch)
    }

    t1 := tree.New(1)
    t2 := tree.New(2)

    res1 := Same(t1, t1)
    fmt.Println("Res 1: %v", res1)

    res2 := Same(t1, t2)
    fmt.Println("Res 2: %v", res2)
}
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