Sending and Receiving Data

In this lesson, explore sending and receiving data through one small Go program, and connect its output to the language rules behind it.

A function can require a send-only channel (chan<- T) or a receive-only channel (<-chan T). These types express who is responsible for producing and consuming values.

Conversions from a bidirectional channel to directional parameters prevent accidental misuse in a helper function.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func produce(out chan<- int)    { out <- 21 }
func consume(in <-chan int) int { return (<-in) * 2 }
func main() {
	values := make(chan int)
	go produce(values)
	fmt.Println(consume(values))
}

Output

42

produce can only send, while consume can only receive. The transmitted 21 is doubled to 42.

• A send-only function parameter cannot receive from its channel.
• Avoid letting many goroutines close a channel they do not own.

Type a channel pipeline with two helpers: one produces a number and the other prints it.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Sending and Receiving Data in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.