I actually like `#<void>`, because it makes more of a distinction between pure and impure functions.
I read a good blog post[0] recently on how not distinguishing makes it difficult to guess the behaviour of simple and short code snippets (in JavaScript, but the same could apply to Arc).
There's definitely a tension between being a concise language and being a safe language. Arc doesn't try to help newcomers avoid simple mistakes. It gives them enough rope to hang themselves, like with unhygienic macros. That's partly why I stopped using Arc to teach my students programming (http://akkartik.name/post/mu).
