Function Expressions

Functions can be also used as expressions. The syntax is the same as for function declarations, except that function expressions have no name, i.e., they are anonymous.

// Declare a constant named `double`, which has a function as its value.
//
// The function multiplies a number by two when it is called.
//
// This function's type is `((Int): Int)`.
//
let double =
    fun (_ x: Int): Int {
        return x * 2
    }