A simply way of DRYing up promise-returning functions

promises.coffee

# There's an incredibly common pattern when using promises:
#
#     myFunction: ->
#       dfd = $.Deferrred()
#
#       $.ajax "wherever"
#       .done() ->
#         dfd.resolve("yep")
#       .fail() ->
#         dfd.reject("nope")
#
#       dfd.promise()
#
# The particularly repetative bits are:
#
#     dfd = $.Deferred()
#     # ...
#     dfd.promise()
#
# It's a little tiresome to constantly create deferreds and return their promises, so here's
#   a conviencince method which does that much for you:
#
#     myFunction: promises (dfd) ->
#      $.ajax "whatever"
#      .done ->
#        dfd.resolve()
#      .fail ->
#        dfd.reject()
#
# It works with arguments, and in class declarations:
#
#     class MyClass
#       showName: promises (dfd, name) ->
#         setTimeout (-> alert(name); dfd.resolve()), 1000
#
# The only wrinkle is that it hides the parent class from CoffeeScript's `super` keyword, so you
#   have to use an alternate syntax:
#
#     class MyChildClass extends MyClass
#       showName: (name) ->
#         promises (dfd) =>
#           super

exports = this
exports.promises = (callback) ->
  (args...) ->
    dfd = $.Deferred()
    args.unshift(dfd)
    callback.apply(@, args)
    dfd.promise()