Channel Directions

When using channels as function parameters, you can specify if a channel is meant to only send or receive values. This specificity increases the type-safety of the program.

package main

import "fmt"

This ping function only accepts a channel for sending values. It would be a compile-time error to try to receive on this channel.

func ping(pings chan<- string, msg string) {
	pings <- msg
}

The pong function accepts one channel for receives (pings) and a second for sends (pongs).

func pong(pings <-chan string, pongs chan<- string) {
	msg := <-pings
	pongs <- msg
}

func main() {
	pings := make(chan string, 1)
	pongs := make(chan string, 1)
	ping(pings, "passed message")
	pong(pings, pongs)
	fmt.Println(<-pongs)

A bidirectional channel converts to either direction automatically, but not back.

	var recvOnly <-chan string = pongs
	fmt.Printf("%T\n", recvOnly)

	// recvOnly <- "x"
	// error: invalid operation: cannot send to receive-only channel <-chan string recvOnly (variable of type <-chan string)
}

$ go run ./36_channel_directions
passed message
<-chan string

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