Monoidal Cancellations (ported from Swift Sandbox)

monoidal-cancellation.swift

//  Write some awesome Swift code, or import libraries like "Foundation",
//  "Dispatch", or "Glibc"
struct Monoid<T> {
    let empty: T
    let append: (T, T) -> T
    
    func fold(_ arr: [T]) -> T {
        return arr.reduce(empty, append)
    }
    
    func foldMap<R>(_ arr: [R], _ f: (R) -> T) -> T {
        return fold(arr.map(f))
    }
}

// TODO: Atomic this
func once(_ f: @escaping () -> ()) -> () -> () {
    var ran = false
    return {
        if !ran {
            f()
            ran = true
        }
    }
}

struct CancelToken {
    static let monoid: Monoid<CancelToken> =
        Monoid(empty: CancelToken(cancel: {}), append: { f, g in 
            CancelToken(cancel: { f.cancel(); g.cancel() }) 
        })

    private let fn: () -> ()
    init(cancel: @escaping () -> ()) {
        self.fn = once(cancel)
    }
    
    func cancel() {
        fn()
    }
}

let c1 = CancelToken(cancel: { print("cancel 1") })
let c2 = CancelToken(cancel: { print("cancel 2") })
CancelToken.monoid.fold([c1, c2, c1, c2]).cancel()