Thunk: a Bluebird Promise whose resolver is only started when its value is needed

Thunk.coffee

class Thunk extends Promise
  evaluated: false
  constructor: (resolver) ->
    deferred = Promise.defer()
    @eval = ->
      new Promise(resolver).then(
        deferred.resolve
        deferred.reject
      )
    super deferred.promise
  _addCallbacks: ->
    if not @evaluated
      @evaluated = true
      @eval()
    super arguments...