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The Way to Go

Functions as Return Variables

Explore how to return functions as variables in Go through closures. Understand creating incrementers, non-recursive Fibonacci functions, and factory functions that generate reusable behavior. This lesson helps you master advanced functional techniques useful for efficient Go programming and code organization.

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Returning a function using closures

Just like we return a variable or a value from a function, we can return a function too. The following function genInc returns a function:

// genInc creates an "increment n" function
func genInc(n int) func(x int) int {
    return func(x int) int {
        return x+n
    }
}

It’s obvious from the header of genInc, that it’s returning a function that takes a parameter x of type int, and that function returns an int value. The function returned by genInc returns x+n. The following program is an implementation of returning a function.

Go (1.6.2)
package main
import "fmt"
func main() {
    // make an Add2 function, give it a name p2, and call it:
    p2 := Add2()
    fmt.Printf("Call Add2 for 3 gives: %v\n", p2(3))
    // make a special Adder function, a gets value 3:
    TwoAdder := Adder(2)
    fmt.Printf("The result is: %v\n", TwoAdder(3))
}
func Add2() (func(b int) int) {         // return a function
    return func(b int) int {
        return b + 2
    }
}
func Adder(a int) (func(b int) int) {   // return a function
    return func(b int) int {
        return a + b
    }
}

In the above program, we implement two functions Add2 and Adder, which return another lambda function. There is a header of Add2 at line 13. You can notice that it returns an anonymous function that takes b (of type int) as a parameter and returns an int value. That anonymous function is returning b+2. Similarly, there is a header Adder at line 18. You can notice that it takes a parameter a (of type int), and it also returns a closure that takes b (of type int) as a parameter and returns an int value. That closure is returning a+b.

Now, look at line 6 in main. We are calling Add2 and setting it equal to p2. Then, we call p2(3) at line 7, which calls the closure returned by Add2. So b of Add2 is equal to 3. On returning b+2, 5 will be printed. Similarly, look at line 9 in main. We are calling Adder(2) and setting it equal to TwoAdder. Then, we call TwoAdder(3) at line 10, which calls the closure returned by Adder. So b of Adder is equal to ...