Simple Decorator

decorator.coffee

class Decorator
  constructor: ({pre, post}) ->
    return (func) ->
      (args...) ->
        args = pre?(args...) or args
        ret = func(args...)
        post?(ret) or ret
 
      
currency = new Decorator({post : (a) -> '$' + a})

tax = currency (x) ->  x * 0.2

console.log tax 100