minimal-promise.js

module.exports = { Promise: Promise, map: map, chain: chain, either: either, cache: cache }

// minimal promises.js
function map(promise, lambda) {
  return Promise(function (resolve, reject) {
    promise.then(function (x) { resolve(lambda(x)) }, reject)
  })
}

function chain(promise, lambda) {
  return Promise(function (resolve, reject) {
    promise.then(function (value) {
      lambda(value).then(resolve, reject)
    }, reject)
  })
}

function either(promise, recover, lambda) {
  return Promise(function (resolve, reject) {
    promise.then(function (value) {
      lambda ? lambda(value).then(resolve, reject) : resolve(value)
    }, function (error) {
      recover(error).then(resolve, reject)
    })
  })
}

function cache(promise) {
  var cached, resolves, rejects

  return Promise(function (resolve, reject) {
    if (cached) {
      return (cached.v ? resolve(cached.v) : reject(cached.e))
    } else if (resolves) {
      return resolves.push(resolve), rejects.push(reject)
    }

    resolves = [resolve], rejects = [reject]
    promise.then(function (value) {
      cached = { v: value } && resolves.forEach(function (r) { r(value) })
    }, function (error) {
      cached = { e: error } && rejects.forEach(function (r) { r(error) })
    })
  })
}

function Promise(handler) {
  return { then: function (f, r) { handler(f, r) } }
}