Prism.swift

struct Prism<S, A> {
    private let getOption: (S) -> A?
    private let reverseGet: (A) -> S

    init(getOption: @escaping (S) -> A?, reverseGet: @escaping (A) -> S) {
        self.getOption = getOption
        self.reverseGet = reverseGet
    }
}

func somePrism<A>() -> Prism<A?, A> {
    return Prism<A?, A>(
        getOption: { optA in
            return optA
        },
        reverseGet: { a in
            return .some(a)
        }
    )
}

func nonePrism<A>() -> Prism<A?, Void> {
     return Prism<A?, Void>(
        getOption: { optA in
            switch optA {
            case .none: return .some(())
            case .some(let a): return .none
            }
        },
        reverseGet: { _ in
            return .none
        }
    )   
}