Recoverable

LazyRecoverable.swift

func const<A, B>(_ a: A) -> (B) -> A {
    return { _ in a }
}

struct LazyRecoverable<A> {
    private let original: A
    private var updatef: ((A) -> A)?
    private var mutableCopy: A

    init(_ original: A) {
        self.original = original
        self.mutableCopy = original
    }

    mutating func update(_ f: @escaping (A) -> A) {
        if let updatef = self.updatef {
            self.mutableCopy = updatef(mutableCopy)
        }
        self.updatef = f
    }

    func commit() -> A {
        let f = updatef ?? const(mutableCopy)
        return f(mutableCopy)
    }

    func rollback() -> A {
        return original
    }
}

Recoverable.swift

struct Recoverable<A> {
    private let original: A
    private var mutableCopy: A

    init(_ original: A) {
        self.original = original
        self.mutableCopy = original
    }

    mutating func update(_ f: (A) -> A) {
        self.mutableCopy = f(mutableCopy)
    }

    func commit() -> A {
        return mutableCopy
    }

    func rollback() -> A {
        return original
    }
}