Golang : Function as an argument type example

When we need to invoke another function inside a.... let's say outer function, the typical way to do this is to call the desire function inside the outer function. For example, fmt.Sprintf() :

 func int2str(i int) string {
 str := fmt.Sprintf("%d", i)
 }

In Golang, we are allowed to pass a function as an argument of another function. This can be illustrated by comparing the differences between these two simple programs.

Program 1:

 package main

 import (
 "fmt"
 "strconv"
 )

 func OuterFunc(a int, b int) string {
 c := a + b
 return strconv.Itoa(c)
 }

 func main() {
 result := OuterFunc(1, 2)
 fmt.Println(result)
 }

versus

Program 2:

 package main

 import (
 "fmt"
 "strconv"
 )

 var a string = "1"

 func OuterFunc(strToInt func(s string) int, b int) string {
 c := strToInt(a) + b
 return strconv.Itoa(c)
 }

 func main() {

 strToInt := func(s string) int {
 num, _ := strconv.Atoi(s)
 return num
 }

 result := OuterFunc(strToInt, 2)
 fmt.Println(result)
 }

Both programs will produce the same output.

This is also known as anonymous function and you can read a better example at https://gobyexample.com/closures

Now, why use a function as an argument of another function?

The motivation behind this ability is to allow you to specify and pass logic(algorithm) to another function. This is a powerful feature to have in programming language. One clear example that I can think of is how bytes.Map() function uses this feature to handle the mapping function.

Code from https://www.socketloop.com/references/golang-bytes-map-function-example

 package main

  import (
 "bytes"
 "fmt"
  )

  func main() {
 str := []byte("abcxefg")

 mapping := func(replacekey rune) rune {
 if replacekey == 'x' {
 return 'd'
 }
 return replacekey
 }

 fmt.Println(string(str))

 // now replace x with d

 fmt.Println(string(bytes.Map(mapping, str)))

  }

Output:

abcxefg

abcdefg

To see the inner working of how bytes.Map() function works, check out https://golang.org/src/bytes/bytes.go?s=8894:8946#L347

To know more about how Golang treat functions, please read this article - "First Class Functions in Go" by Andrew Gerrand.

Happy coding!

References:

https://golang.org/pkg/bytes/#Map

https://www.socketloop.com/references/golang-bytes-map-function-example

https://golang.org/src/bytes/bytes.go?s=8894:8946#L347