phase2.swift

import Foundation

enum Result<T> {
    case success(T)
    case failure(String)

    func map<U>(_ f: (T) -> U) -> Result<U> {
        switch self {
        case .success(let x): return .success(x |> f)
        case .failure(let e): return .failure(e)
        }
    }
}

func calculator(_ input: String) -> Double {
    return input |> split |> reversePolishNotation
}

func reversePolishNotation(_ input: [String]) -> Double {
    let array = input.reduce([], _reversePolishNotation)
    return array.first!
}

func _reversePolishNotation(_ input: [Double], _ s: String) -> [Double] {
    switch s {
    case "+":
        return [input[0] + input[1]] + input[2..<input.count]
    default:
        if let double = Double(s) {
            return [double] + input
        }
        return input
    }
}

func fake<T>(_ input: T) -> Double { return 0 }

func split(_ input: String) -> [String] {
    return input.components(separatedBy: " ")
}

calculator("4 5 + 3 +")