Tour of Go 2.0

Function closures

A closure is a function that captures variables from the scope around it. The function can read and update those variables — and they persist between calls.

In the example, adder returns a closure that accumulates into its own sum. Each call to adder() creates an independent closure with its own sum.

Try it: Notice that pos and neg each have separate state — they don’t share sum.

main.go ▶ Run in Playground 
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),
		)
	}
}