Future and ReaderT with auto-lifting and example

01-future.js

class Future {
  constructor(computation) {
    this.fork = computation
    this.__future__ = true;
  }
  static is(val) {
      return (val != null && val.__future__ === true)
  }
  static of(value) {
    if (Future.is(value)) return value;
    return new Future((resolve, reject) => resolve(value))
  }
  static err(e) {
    if (Future.is(e)) return e;
    return new Future((resolve, reject) => reject(e))
  }
  chain(f) {
    return new Future((resolve, reject) =>
      this.fork(value => Future.of(f(value)).fork(resolve, reject), reject))
  }
  orElse(f) {
    return new Future((resolve, reject) =>
      this.fork(resolve, error => Future.of(f(error)).fork(resolve, reject)))
  }
}

02-readert.js

var ReaderT = (M) => class ReaderT {
  constructor(computation) {
    this.computation = computation
    this.__reader__ = true;
  }
  static is(val) {
      return (val != null && val.__reader__ === true)
  }
  runWith(data) {
    return this.computation(data)
  }
  chain(f) {
    return new ReaderT(data => this.computation(data).chain(a => ReaderT.of(f(a)).runWith(data)));
  }
  static of(val) {
    if (ReaderT.is(val)) return val;
    return new ReaderT(data => M.of(val))
  }
  static ask() {
    return new ReaderT(data => M.of(data))
  }
}

03-example.js

var fs = require('fs')
var readFile = path => new Future((resolve, reject) => fs.readFile(path, 'utf8', (err, res) => err ? reject(err) : resolve(res)))


var SuperFuture = ReaderT(Future)

var doStuffx = () => getUserId().chain(id => readFile(id + '.txt'))
var getUserId = () => SuperFuture.ask().chain(user => user.id)



// From the route handler, we only pass the user id once.
// It automatically propagates through the SuperFutures
app.get('/some/url', function(req, res) {
  doStuffx().runWith({id: req.user.id}).fork(data => res.end(data), err => console.error(err))  
})