LReader (LIOのReader版)

LReader.swift

struct LReader<S1, S2, E, A> {
    let run: (E) -> A

    init(_ run: @escaping (E) -> A) {
        self.run = run
    }

    func map<B>(_ f: @escaping (A) -> B) -> LReader<S1, S2, E, B> {
        return LReader<S1, S2, E, B> { env in f(self.run(env)) }
    }

    func flatMap<B, S3>(_ f: @escaping (A) -> LReader<S2, S3, E, B>) -> LReader<S2, S3, E, B> {
        return LReader<S2, S3, E, B> { env in f(self.run(env)).run(env) }
    }
}