gistfile1.swift

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

import UIKit

protocol StageProtocol {
    associatedtype StageIn
    associatedtype StageOut
}

class Stage<In, Out> : StageProtocol {
    typealias StageIn = In
    typealias StageOut = Out
}

protocol StageHeadProtocol : StageProtocol {}

class StageHead<In, Out> : Stage<In, Out>, StageHeadProtocol {
    let mapper: (In) -> Out
    
    init(mapper: @escaping (In) -> Out) {
        self.mapper = mapper
    }
}

protocol IntermediateStageProtocol : StageProtocol {
    associatedtype PrevStage : StageProtocol
    
    
    var previousStage: PrevStage { get }
}

class IntermediateStage<In, Out, PreviousStage: StageProtocol> : Stage<In, Out>, IntermediateStageProtocol where PreviousStage.StageOut == In {
    typealias PrevStage = PreviousStage
    let mapper: (In) -> Out
    
    init(mapper: @escaping (In) -> Out, previousStage: PreviousStage) {
        self.mapper = mapper
        self.previousStage = previousStage
    }
    
    let previousStage: PreviousStage
}

func first<T: StageHeadProtocol>(_ stage: T) -> T {
    return stage
}

func first<T: IntermediateStageProtocol, U>(_ stage: T) -> U where T.PrevStage : StageHeadProtocol, U == T.PrevStage  {
    return stage.previousStage
}

func first<T: IntermediateStageProtocol, U>(_ stage: T) -> U where T.PrevStage : IntermediateStageProtocol, T.PrevStage.PrevStage : StageHeadProtocol, U == T.PrevStage.PrevStage {
    return stage.previousStage.previousStage
}

func first<T: IntermediateStageProtocol, U>(_ stage: T) -> U where T.PrevStage : IntermediateStageProtocol, T.PrevStage.PrevStage : IntermediateStageProtocol, T.PrevStage.PrevStage.PrevStage : StageHeadProtocol, U == T.PrevStage.PrevStage.PrevStage {
    return stage.previousStage.previousStage.previousStage
}


let head = StageHead(mapper: { (x: Int) -> (Int) in return x * x })
first(head).mapper(10)

let next1 = IntermediateStage(mapper: { (x: Int) -> (Int) in return x + 1 }, previousStage: head)
first(next1).mapper(10)

let next2 = IntermediateStage(mapper: { (x: Int) -> (Int) in return x + 42 }, previousStage: next1)
first(next2).mapper(10)

let next3 = IntermediateStage(mapper: { (x: Int) -> (String) in return "\(x)" }, previousStage: next2)
first(next3).mapper(10)