Anonymous Functions in Go

The anonymous function is a feature in Golang which let us define a function without a name. This feature is also called a function literal. This is useful when you want an inline function or to form a closure.

Declaring the anonymous function

The syntax is pretty straightforward and much similar to normal function.

func(parameter_list)(return_type){
  return
}()

Parameter list and return type are optional.

() this will invoke the function as soon as it is defined.

package main

import "fmt"

func main() {
    func() {
        fmt.Println("Golang Rocks!")
    }()
}

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Assigning an anonymous function to a variable

In Go, you can assign an anonymous function to a variable. The assigned variable will be of function type and it can be called as a regular function.

package main

import "fmt"

func main() {
    v := func() {
        fmt.Println("Golang Rocks!")
    }
    fmt.Printf("Type of variable v: %T\n", v)
    v()
}

Type of variable v: func()
Golang Rocks!

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Passing an argument to an anonymous function

An anonymous function can take any number of arguments similar to a regular function.

package main

import "fmt"

func main() {
    func(name string) {
        fmt.Println("Hello, ", name)
    }("Jack")
}

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With any number of trailing arguments similar to Variadic functions

package main

import "fmt"

func main() {
    func(i ...int) {
        fmt.Println(i)
    }(1, 2, 3, 4)
}

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Passing an anonymous function as an argument

You can pass an anonymous function as an argument to a regular function or an anonymous function.

Anonymous functions as an argument to a regular function

package main

import "fmt"

func sayHello(af func(s string) string) {
    fmt.Println(af("Jack"))
}

func main() {

    sayHello(func(s string) string {
        return "Hello, " + s
    })
}

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Anonymous function as an argument to an anonymous function

package main

import "fmt"

func main() {
    func(v string) {
        fmt.Println(v)
    }(func(s string) string {
        return "Hello, " + s
    }("Jack"))
}

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This above code looks a bit complicated and hard to fathom so another way we can achieve this is the following:

package main

import "fmt"

func main() {
    af := func(s string) string {
        return "Hello, " + s
    }

    func(af func(s string) string) {
        fmt.Println(af("Jack"))
    }(af)
}

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Return an anonymous function from another function

We can return an anonymous function from another function

Returning from regular function

package main

import "fmt"

func sayHello() func(s string) string {
    r := func(s string) string {
        return "Hello, " + s
    }
    return r
}

func main() {
    f := sayHello()
    fmt.Printf("Type of variable f: %T\n", f)
    fmt.Println(f("Jack"))
}

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Returning from the anonymous function

package main

import "fmt"

func main() {
    f := func() func(s string) string {
        r := func(s string) string {
            return "Hello, " + s
        }
        return r
    }

    fmt.Printf("Type of variable f: %T\n", f)
    c := f()

    fmt.Printf("Type of variable c: %T\n", c)
    fmt.Println(c("Jack"))
}