Swift Shields

Shield.swift

// Creates a new function from an inout function
// that does not modify the passed in arugment
// but returns a modified argument and result
func shield<T,U>(action: inout T -> U) -> T -> (T, U) {
    return { arg in
        var copy = arg
        let result = action(&copy)
        return (copy, result)
    }
}

// Example

var arr = [1,2,3]

let last = shield(removeLast)(arr)

let removeFirst = { (inout x : Array<Int>) in removeAtIndex(&x, 0) }
let first = shield(removeFirst)(arr)

println(arr)   // -> [1, 2, 3]
println(last)  // -> ([1, 2], 3)
println(first) // -> ([2, 3], 1)

// Note that the value of arr never changed!