do.coffee

# Do comprehension chains Monads together
var x = do
  a <- maybe_null( 'Hello World' ) # If returns null/None, none of the following is executed
  b <- async_call_1( a ) # Returns a Promise
  c <- async_call_2( b ) # Only executed once the preceding promise is fulfilled
  d <- async_call_3( c )
  return d

x.get(function(err,ok){
  # function called with err if the above comprehension failed
  # called with ok=d if the above comprehension succeeded 
})