Except isn't that missing the actual point? The example was contrived to drive home that we can use objects as functions in ClojureScript - I respect that the engaged reader understands that these tools must be applied tastefully. I could have implemented it just like you've shown but would we have learned anything? :)

Also my example is one step from being open, extensible to any type instead of hard coding string/array which you copied. You can't do this in CoffeeScript in a safe way:

  (deftype Slice [start end]
      ISlice
      (-shift [_] (Slice. (inc start) (inc end)))
      IFn
      (-invoke [_ x]
        (-slice x start end)))