Interfaces / Traits / Multiple inheritance

[cbart]

As far as I can tell CoffeeScript supports only single inheritance (as for example Java). Multiple inheritance on the other hand can be painful, but it would be very nice if it supported traits (like in Scala - maybe not that powerfull, but kind of).
To be clear traits are like java interfaces, but unlike them - they provide implementation and attributes inside. However, like the Java interfaces, we cannot have actual instance of a trait. Also they are (in practice) quite like Python's mixins. For example:

class Vehicle
    constructor: (@power) ->
    getPower: ->
        @power

trait TurboBoost
    getPower: ->
        2 * super()

class Ferrari extends Vehicle with TurboBoost
    getPower: ->
        super() + 3

ferrari = new Ferrari 18
ferrari.getPower()

###
this would give: 2 * (3 + 18) = 42
###

There could be simple implementation of them as simple JS objects (like 'modules' in Prototype toolkit, which are mixed into classes at creation time).

[jashkenas]

This is a duplicate of the following tickets...

http://github.com/jashkenas/coffee-script/issues/issue/452
http://github.com/jashkenas/coffee-script/issues/issue/218

The reason why we don't have language support for mixins in CoffeeScript is because they're not supported by JavaScript -- you can't actually have more than one prototype chain on an object, meaning that the mixin's properties are simply copied over, which breaks both inheritance, and subsequent changes to the mixin. If you'd like to mix things in yourself, I recommend an extends function.