result_monad.swift

//: Playground - noun: a place where people can play

import Darwin
import Foundation

enum Result<T> {
    case Ok(T)
    case Err(String)
}

infix operator >>> { associativity left }
func >>> <A, B, C>(f: B -> C, g: A -> B) -> A -> C {
    return { x in f(g(x)) }
}

func gamble(probability:UInt32) -> Bool {
    return probability > UInt32(arc4random_uniform(101))
}

func task_1(input:Result<String>) -> Result<String> {
    switch input {
    case .Ok(let string):
        if gamble(30) {
            return Result.Err("Task 1 error, string: " + string)
        } else {
            return Result.Ok(string.uppercaseString)
        }
    case .Err(let string):
        return Result.Err(string)
    }
}

func task_2(input:Result<String>) -> Result<String> {
    switch input {
    case .Ok(let string):
        if gamble(30) {
            return Result.Err("Task 2 error, string: " + string)
        } else {
            return Result.Ok("OUTPUT")
        }
    case .Err(let string):
        return Result.Err(string)
    }
}

func task_3(input:Result<String>) -> Result<String> {
    switch input {
        case .Ok(let string):
            if gamble(30) {
                return Result.Err("Task 3 error, string: " + string)
            } else {
                return Result.Ok(string.capitalizedString)
            }
        case .Err(let string):
            return Result.Err(string)
    }
}

let pipeline = task_3 >>> task_2 >>> task_1

for index in 1...10 {
    print(pipeline(Result.Ok("Input")))
}