Closures / Anonymous Functions

// Anonymous function (immediately invoked)
func() {
    // body
}()

// Assign to a variable.
add := func(a, b int) int {
    return a + b
}
result := add(3, 4) // 7

// Closure (captures a variable from the outer scope)
counter := 0
increment := func() int {
    counter++ // references the outer variable counter
    return counter
}

// Higher-order function that returns a function (closure factory)
func makeAdder(x int) func(int) int {
    return func(y int) int {
        return x + y // x is captured by the closure
    }
}

package main

import "fmt"

// Factory function that creates a counter.
func makeCounter(start int) func() int {
    count := start
    return func() int {
        count++ // count is captured by the closure
        return count
    }
}

// Factory function that creates a multiplier function.
func multiplier(factor int) func(int) int {
    return func(n int) int {
        return n * factor
    }
}

// Higher-order function that accepts a function as an argument.
func apply(nums []int, fn func(int) int) []int {
    result := make([]int, len(nums))
    for i, n := range nums {
        result[i] = fn(n)
    }
    return result
}

func main() {
    // Encapsulate state with closures.
    counter1 := makeCounter(0)
    counter2 := makeCounter(10) // each has its own independent state
    fmt.Println(counter1()) // 1
    fmt.Println(counter1()) // 2
    fmt.Println(counter2()) // 11
    fmt.Println(counter1()) // 3 (not affected by counter2)

    fmt.Println()

    // Generate customized functions using a factory.
    double := multiplier(2)
    triple := multiplier(3)
    nums := []int{1, 2, 3, 4, 5}
    fmt.Println("doubled:", apply(nums, double))
    fmt.Println("tripled:", apply(nums, triple))

    fmt.Println()

    // Pass an anonymous function inline.
    fmt.Println("squared:", apply(nums, func(n int) int {
        return n * n
    }))

    // Initialization with an immediately invoked function
    result := func(a, b int) int {
        return a + b
    }(100, 200)
    fmt.Println("immediately invoked result:", result)
}

In Go, functions are treated as first-class values. Because a closure holds a reference to the captured variable, any changes made through the closure affect the original variable. This behavior enables a design similar to object-oriented encapsulation.

A well-known bug occurs when launching goroutines inside a loop: the closure may not capture the loop variable correctly. Pass the loop variable as an argument to the closure, or create a local copy inside the loop body. Note that Go 1.22 changed how loop variables are scoped.

For variadic arguments, see Variadic Arguments. For combining closures with goroutines, see Goroutine.