Simple Swift Function to memoize generic functions of the form T -> U

MemoizationWithGenerics.swift

func memoized<Input:Hashable,Output>(fn : @escaping (Input) -> Output) -> (Input)->Output {
    var cache = [Input:Output]()
    let function = fn
    let closure = {
      (val : Input) -> Output in
      let value = cache[val]
      if value != nil {
        return value!
      } else {
        let newValue = function(val)
        cache[val] = newValue
        return newValue
      }
    }
    
    return closure
}

func fibonacci(_ value:Int) -> Int {
    var result = 0
    switch value {
        case _ where value < 0:
            result = 0
        case 0:
            result = 0
        case 1,2:
            result = 1
        default:
            result = fibonacci(value-1) + fibonacci(value-2)
    }
    return result
}

let fastFibo = memoize(fibonacci)
fastFibo(90)

let fastSquare = memoize({(x:Int) -> Int in return x*x})
fastSquare(900000)
fastSquare(900000)

Please note that this does not work well for memoizing the fibonacci function because it is recursive.