A solution using functional programing.swift

func cached<In: Hashable, Out>(_ f: @escaping (In) -> Out) -> (In) -> Out{
    var cache = [In: Out]()
    
    return { (input: In) -> Out in
        if let cachedValue = cache[input] {
            return cachedValue
        } else {
            let result = f(input)
            cache[input] = result
            return result
        }
    }
}