> in function header definition

Hmmm, I can kind of see it, but then you'd also need function overloading, right? With this you'd just do outer/inner functions, where the outer determines the pattern match and dispatches along to the appropriate inner function.

If you want to just make sure the function only accepts arguments that match a pattern, you're basically talking a guard pattern. Maybe a `return unless foo in <pattern>` syntax would then be nice (or some other one-liner), but it's not like it'd be hard to do it using this feature, you'd just use a few more mostly empty lines.