nested-monads.js

const Future = require('fluture');
const R = require('ramda');
const Reader = require('./monads/reader');

// Setup
const FakeDb = () => {
  const data = {
    1: {id: 1, name: 'bob', bff: 2},
    2: {id: 2, name: 'alice', bff: 1}
  };

  const byId = id => Future.of(data[id]);
  const save = (id, value) => {
    data[id] = value;
    return Future.of(value);
  };

  return { byId, save };
};
const upperName = data => data.name.toUpperCase();

const raw = () => {
  const program = Reader(db => db.byId(1))
    .chain(future => {
      return Reader(config => {
        return future.chain(data => {
          return Reader(db => db.byId(data.bff)).run(config);
        });
      });
    })
    .map(responseFuture => responseFuture.map(upperName));

  program.run(FakeDb()).value(console.log);
}

const monadTransformer = () => {
  const ReaderT = value => {
    const map = fn => ReaderT(value.map(future => future.map(fn)));
    const chain = fn => ReaderT(value.chain(future => {
      return Reader(req => {
        return future.chain((...args) => {
          return fn(...args).run(req);
        });
      });
    }));

    return Object.freeze({ map, chain, run: value.run });
  }

  const program = ReaderT(Reader(db => db.byId(1)))
    .chain(data => ReaderT(Reader(db => db.byId(data.bff))))
    .map(upperName)

  program.run(FakeDb()).value(console.log);
}

raw();
monadTransformer();