fusion.swift

//
//  main.swift
//  Deforestation
//
//  Created by Chris Eidhof on 29.03.19.
//  Copyright © 2019 objc.io. All rights reserved.
//
import Foundation

struct Build<Element, Result> {
    let empty: Result
    let append: (inout Result, Element) -> ()
}

struct Lazy<Element, Result> {
    let force: (_ consumer: Build<Element, Result>) -> Result
    func filter(_ cond: @escaping (Element) -> Bool) -> Lazy<Element, Result> {
        return Lazy { consumer in
            self.force(Build(empty: consumer.empty, append: { arr, el in
                if cond(el) {
                    consumer.append(&arr, el)
                }
            }))
        }
    }
    
    func map<B>(_ transform: @escaping (Element) -> B) -> Lazy<B, Result> {
        return Lazy<B, Result> { consumer in
            self.force(Build(empty: consumer.empty, append: { arr, el in
                consumer.append(&arr, transform(el))
            }))
        }
    }
    
    func reduce(_ initial: Result, _ combine: (inout Result, Element) -> ()) -> Result {
        return withoutActuallyEscaping(combine) { comb in
            force(Build(empty: initial, append: comb))
        }
    }
}

extension Array {
    func lazy<Result>() -> Lazy<Element, Result> {
        return Lazy { witness in
            var result = witness.empty
            for i in self {
                witness.append(&result, i)
            }
            return result
        }
    }
    
}

print([1,2,3].lazy().map({ $0 * 2}).filter { $0 != 0 }.reduce(0, +=))
print([1,2,3].lazy().map({ $0 * 2}).filter { $0 != 0 }.reduce([], { $0.append($1) }))