existentials

existentials.swift

indirect enum Parser<A> {
    case nilP(v: A?)
    case altP(p1: Parser<A>, p2: Parser<A>)
    case _bindP(p1: Parser<Any>, cont: (Any) -> Parser<A>) // don't use directly

    func map<B>(_ f: @escaping (A) -> B) -> Parser<B> {
        switch self {
        case let .nilP(v: x): return .nilP(v: x.map(f))
        case let .altP(p1: p1, p2: p2): return .altP(p1: p1.map(f), p2: p2.map(f))
        case let ._bindP(p1: p1, cont: g): return Parser<B>._bindP(p1: p1, cont: { (x: Any) -> Parser<B> in g(x).map(f) })
        }
    }
    
    static func bind<I>(p1: Parser<I>, p2: @escaping (I) -> Parser<A>) -> Parser<A> {
        return ._bindP(p1: p1.map { $0 }, cont: { (x: Any) -> Parser<A> in
            let value = x as! I
            return p2(value)
        })
    }
}