Reader monad Swift 4

gistfile1.swift

class Reader<E, A> {
    let g: (E) -> A
    init(g: @escaping (E) -> A) {
        self.g = g
    }
    func apply(e: E) -> A {
        return g(e)
    }
    func map<B>(f: @escaping (A) -> B) -> Reader<E, B> {
        return Reader<E, B>{ e in f(self.g(e)) }
    }
    func flatMap<B>(f: @escaping (A) -> Reader<E, B>) -> Reader<E, B> {
        return Reader<E, B>{ e in f(self.g(e)).g(e) }
    }
}