Ever since I wrote this I realized that this feature, as outlined here, is not opt-in, even in a vacuum, due to the fact that will break all variables named 'await' and 'defer'.
About using others' code: compile time code generation is always possible, and you can't prevent the libraries you are using from using it. It can even be worse: they can runtime code generation, or a downright obfuscator.
Since more and more languages are being compiled to JS (Clojurescript being one formidable recent addition), trying to stay "pure" with respect to the libraries you use is getting harder and harder.
My biggest worry is that Coffeescript starts stagnating from lack of innovation (which seems to affect most stable languages).
One way Scala managed to experiment with features while still trying to be stable, was through Scala Compiler Plugins[1].
This we could be having a less theoretical conversation, by throwing it in the wild, and learning from the experiments people make with it.
About using others' code: compile time code generation is always possible, and you can't prevent the libraries you are using from using it. It can even be worse: they can runtime code generation, or a downright obfuscator.
Since more and more languages are being compiled to JS (Clojurescript being one formidable recent addition), trying to stay "pure" with respect to the libraries you use is getting harder and harder.
My biggest worry is that Coffeescript starts stagnating from lack of innovation (which seems to affect most stable languages).
One way Scala managed to experiment with features while still trying to be stable, was through Scala Compiler Plugins[1].
This we could be having a less theoretical conversation, by throwing it in the wild, and learning from the experiments people make with it.
[1] http://www.scala-lang.org/node/140