In this example, the save method produces an error and we use mapLower to call a special Future function "mapRej", which transforms all incoming errors.

reader-t.js

const {tagged} = require('daggy');

// const
const K = (a) => (b) => a;

// Reader Transformer
module.exports = (M) => {
  const ReaderT = tagged('run');

  ReaderT.lift = ReaderT.prototype.lift = (m) => ReaderT(K(m));

  ReaderT.of = ReaderT.prototype.of = (a) => ReaderT((e) => M.of(a));

  ReaderT.ask = ReaderT.prototype.ask = ReaderT((e) => M.of(e));

  ReaderT.prototype.chain = function(f) {
    return ReaderT((e) => {
      return this.run(e).chain((a) => f(a).run(e));
    });
  };

  ReaderT.prototype.map = function(f) {
    var m = this;
    return m.chain(a => m.of(f(a)))
  }

  ReaderT.prototype.mapLower = function(f) {
    return ReaderT(a => f(this.run(a)));
  }

  return ReaderT;
};

readert-future-with-error.js

/**
 * This short program will encrypt the user password
 * and insert a new record into a mock database.
 */
const ReaderT = require('./lib/reader-t');
const Future = require('fluture');
const R = require('ramda');
const crypto = require('crypto');
const FutureReader = ReaderT(Future);

// our mock database
const database = [
  { email: 'user@example.org', password: 'e0538fd8f022bb3b139d72cf12766cb0e31690ff' },
  { email: 'admin@example.org', password: '42c4fbf6fec201c66b82c97833b08d936d2cd526' }
]

// creates a statefull database connection
const connectTo = (db) => {
  return {
    insert: (doc) => Future.of(db.push(doc)),
    get: (i) => Future.of(db[i]),
    delete: (i) => Future.of(db.splice(i, 1)),
    list: () => Future.of(db)
  }
}

// some utility functions
const encrypt = (i) => crypto.createHash('sha1').update(i).digest('hex');
const encPassword = R.evolve({password: encrypt})


// utility function which makes it easier
// to ask for some dependency
// first parameter is the selector function which
// is applied to the environment to get a specific
// dependency (inject(R.identity, f, reader) returns the whole
// environment)
const inject = R.curry((sel, f, reader) => {
  return reader.chain((v) => {
    return reader.ask.chain((env) => {
      const dep = sel(env);
      return reader.lift(f(dep, v));
    });
  });
});

// utility function which invokes mapLower on the object
const mapLower = R.invoker(1, 'mapLower');

// this is how you access the db connection inside the reader
const save = (db, user) => {
  //return db.insert(user).chain(_ => db.list());
  return Future.reject('Some error just happened.');
}

// this utility function is designed to work inside
// mapLower call and convert all incoming errors to uppercase
// it is using the special mapRej function of Future
const shoutErrors = Future.mapRej(R.toUpper);

// the body of the program
const handler = R.pipe(
  R.map(encPassword),
  inject(R.prop('db'), save),
  R.map(R.map(R.dissoc('password'))),
  mapLower(shoutErrors)
);

// this is our db connection now
const dbCon = connectTo(database);

const request = { email: 'new@example.org', password: 'secret' };

const result = handler(FutureReader.of(request))
  .run({ 'db': dbCon }) // this is how you pass the db connection in
  .fork(console.error, console.log); // now fork the future