Closures

A closure is a function value that references variables from outside its own body. makeMultiplier returns a new function each time it's called, and each returned function remembers its own factor, captured from the moment it was created.

main.go

Try it yourself →

package main

import "fmt"

func makeMultiplier(factor int) func(int) int {
	return func(n int) int {
		return n * factor
	}
}

func main() {
	double := makeMultiplier(2)
	triple := makeMultiplier(3)

	fmt.Println(double(5))
	fmt.Println(triple(5))
}

Output

10
15

Closures can hold mutable state

A closure doesn't just read captured variables — it can also modify them, and that modification persists between calls. makeCounter's count variable lives on inside the returned closure, incrementing every time next() is called.

main.go

Try it yourself →

package main

import "fmt"

func makeCounter() func() int {
	count := 0
	return func() int {
		count++
		return count
	}
}

func main() {
	next := makeCounter()
	fmt.Println(next())
	fmt.Println(next())
	fmt.Println(next())
}

Output

1
2
3

Next: goroutines — Go's lightweight, built-in concurrency primitive.

Practice exercise

Write a function makeAdder(x int) func(int) int that returns a closure adding x to its argument. Create addFive := makeAdder(5), then print addFive(10).

← defer, panic, and recover Goroutines →