Memoize 1-parameter functions. Saw something like this in the Advanced Swift WWDC talk. Tried to reproduce it.

memoize.swift

func memoize<T: Hashable, U>(f: ((T -> U), T) -> U) -> (T -> U) {
    var cache = [T: U]()
    var memoized: (T -> U)!
    memoized = { n in
        if cache[n] == nil {
            cache[n] = f(memoized, n)
        }
        return cache[n]!
    }
    return memoized
}

let fibonacci: (Int -> Int) = memoize {
    f, n in return (n == 0 || n == 1) ? n : f(n - 1) + f(n - 2)
}