The Reader monad

reader.js

const { match } = require("natch");

const compose = f => g => x => f(g(x));
const kleisli = bind => fk => gk => compose(bind(fk))(gk);

// The reader module
const Reader = (() => {
  // Constructors
  const Pure = v => ({ t: "pure", v });
  const Get = v => ({ t: "get", v });

  // Misc
  const ask = Get(Pure);
  const addGet = a => bind(b => pure(a + b))(ask);
  const runConst = x =>
    match(
      m => m.t,
      ["pure", ({ v }) => v],
      ["get", ({ v }) => runConst(x)(v(x))]
    );

  // Monad
  const pure = Pure;
  const bind = f =>
    match(
      m => m.t,
      ["pure", ({ v }) => f(v)],
      ["get", ({ v }) => Get(kleisli_(f)(v))]
    );

  const kleisli_ = kleisli(bind);

  return { Pure, Get, ask, addGet, runConst, pure, bind };
})();

const { pure, bind, runConst, addGet } = Reader;

const question = addGet(5);
console.log(runConst(15)(question));