Go Closures will be explained in this post. First we’ll start of with an introduction: functions can contain functions. Then we’ll give an example of a closure and finally a practical app.

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Closure

Nested functions

Go functions can contain functions. Those functions can literally be defined in functions.

The example below outputs “hello world”, but demonstrates this nested functions principle.

func main(){
world := func() string {
return "world"
}
fmt.Print("hello ", world(),"\n")
}

What is a closure?

In the next example, it creates a closure. The closure returns a number that increases each time its called.

You can use the variable x inside the increment function below. The function increment and variable x form the closure.

func main(){
x := 2
increment := func() int {
x++
return x
}
fmt.Println(increment())
fmt.Println(increment())
fmt.Println(increment())
}

x will keep being modified by the increment function. Each time the function increment() is called, x is modified.

A closure is a type of function, that uses variables defined outside of the function itself.

Generator

You can use the idea of closures to make a number generator.

In the example below function makeSequence() returns an anonymous function that generates odd numbers. An anonymous function is just a function without a name.

func makeSequence() func() int {
i:=1
return func() int {
i+=2
return i
}
}

makeSequence() returns a function, that function returns the numeric output.

Then we create a function: sequence generator:

sequenceGenerator := makeSequence()

And use it like this:

fmt.Println(sequenceGenerator())
fmt.Println(sequenceGenerator())

Full code:

package main

import "fmt"

func makeSequence() func() int {
i:=1
return func() int {
i+=2
return i
}
}

func main(){
sequenceGenerator := makeSequence()
fmt.Println(sequenceGenerator())
fmt.Println(sequenceGenerator())
fmt.Println(sequenceGenerator())
}