Anonymous functions and closures

A function literal (or lambda) is a function without a name.

In this example a function literal is passed as the less argument to the sort.Slice function.

func Slice(slice interface{}, less func(i, j int) bool)

people := []string{"Alice", "Bob", "Dave"}
sort.Slice(people, func(i, j int) bool {
	return len(people[i]) < len(people[j])
})
fmt.Println(people)
// Output: [Bob Dave Alice]

You can also use an intermediate variable.

people := []string{"Alice", "Bob", "Dave"}
less := func(i, j int) bool {
	return len(people[i]) < len(people[j])
}
sort.Slice(people, less)

Note that the less function is a closure: it references the people variable, which is declared outside the function.

Closures

Function literals in Go are closures: they may refer to variables defined in an enclosing function. Such variables

• are shared between the surrounding function and the function literal,
• survive as long as they are accessible.

In this example, the function literal uses the local variable n from the enclosing scope to count the number of times it has been invoked.

// New returns a function Count.
// Count prints the number of times it has been invoked.
func New() (Count func()) {
	n := 0
	return func() {
		n++
		fmt.Println(n)
	}
}

func main() {
	f1, f2 := New(), New()
	f1() // 1
	f2() // 1 (different n)
	f1() // 2
	f2() // 2
}

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