CoffeeScript: 配列の差分(diff)を取る一行の関数

list1 = ["green", "red", "blue", "red"]
list2 = ["green", "yellow", "red"]

# 独立した関数を定義する場合
diff = (list1, list2)-> (value for value in list1 when list2.indexOf(value) is -1)

# prototypeを拡張する場合
Array::diff = (array)-> (value for value in this when array.indexOf(value) is -1)

console.log diff(list1, list2)
console.log list1.diff(list2)

IE8以下対応は Array.prototype.indexOf を定義しないとだめ http://qiita.com/suin/items/f08af948279f87d84865#1-1

Array::indexOf or= (item) ->
    for x, i in this
        return i if x is item
    return -1