Maybe T example

MaybT.js

const Fluture= require("fluture")

// We can just get map from of and chain for free everywhere
const deriveFunctor = M => ({ ...M, map: f => M.chain(x => M.of(f(x))) })

// Identity monad
const id = x => x;
const Identity = deriveFunctor({ of: id, chain: id });

const Maybe = (() => {
  // ADT
  const Just = x => ({ hasValue: true, x });
  const Nothing = { hasValue: false };
  const match = ({ Just, Nothing }) => ({ hasValue, x }) =>
    hasValue ? Just(x) : Nothing;

  // Monad transformer
  const T = M => {
    // Monad
    const of = x => M.of(Just(x));
    const chain = f =>
      M.chain(match({
        Nothing: M.of(Nothing),
        Just: f
      }));

    // MonadTrans
    const liftT = M.map(Just);

    return deriveFunctor({ of, chain, liftT })
  };

  // Monad
  const { of, chain, map } = T(Identity)

  return { Just, Nothing, match, T, of, chain, map };
})();

const FlutureOfMaybe = Maybe.T(Fluture)
const tests = [
  FlutureOfMaybe.map(x => x * 2)(Fluture.of(Maybe.Nothing)), // => Fluture.of(Nothing)
  FlutureOfMaybe.map(x => x * 2)(Fluture.of(Maybe.Just(21))) // => Fluture.of(Just(42))
]

tests.map(f => f.fork(console.error, console.log))

// The fact that you can do `const { map, of, chain, ...etc } = Maybe.T(Fluture)` 
// centralizes the complexity of threading maybes through flutures to a single place