Swift Callback Function Transformation

Swift Callback.swift

/*
 * Takes in a function that transform T to U and
 * a callback that takes arguments of type T and U
 * and returns a function that performs f but calls
 * the callback before returning
 */
func callback<T,U>(f: T -> U, c: (T, U) -> ()) -> T -> U{
    return { x in
        let r = f(x)
        c(x,r)
        return r
    }
}

/*
 * Example usage of our callback function
 * that will create a new function that performs
 * f but also prints the input and output of f
 */
func print<T,U>(f: T -> U) -> T -> U {
    return callback(f, { x, r in println("\(x) -> \(r)") })
}

/*
 * Similiar to the print function, but also takes in a
 * tag as input
 */
func print<T,U>(f: T -> U, tag: String) -> T -> U {
    return callback(f, { x, r in println("\(tag): \(x) -> \(r)") })
}

/*
 * Pretty intutive, e.g. identity(x) -> x
 */
func identity<T>(x: T) -> T {
    return x
}

// Example A

var x = 0
for i in 1...4 {
    x = print(+, "add")(0,i)  // -> "add: (0,1) -> 1"
}                             //    "add: (1,2) -> 3"
                              //    "add: (3,3) -> 6"
                              //    "add: (6,4) -> 10"
                  
println("the result is \(x)") // -> "the result is 10"



// Example B

var s = ""
for i in "Hey" {
    s.append(print(identity)(i)) // -> "H -> H"
                                 //    "e -> e"
                                 //    "y -> y"
                              
    print(s.append)(i)           // -> "H -> ()"
}                                // -> "e -> ()"
                                 // -> "y -> ()"

println("the result is \(s)") // "the result is HHeeyy"