The Go Programming Language

5.5 Function Values

Functions are first-class values in Go: like other values, function values have types, and they may be assigned to variables or passed to or returned from functions. A function value may be called like any other function. For example:

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func square(n int) int     { return n * n }
func negative(n int) int   { return -n }
func product(m, n int) int { return m * n }

f := square
fmt.Println(f(3)) // "9"

f = negative
fmt.Println(f(3))     // "-3"
fmt.Printf("%T\n", f) // "func(int) int"

f = product // compile error: can't assign f(int, int) int to f(int) int

The zero value of a function type is nil. Calling a nil function value causes a panic:

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var f func(int) int f(3) // panic: ...