I suspect that you could do this already using typeclasses. We want (-2) to be a function of the type `Num a => a -> a`, so we could, in theory, make functions

I suspect that you could do this already using typeclasses. We want (-2) to be a function of the type `Num a => a -> a`, so we could, in theory, make functions of this type an instance of Num. I do not immediately see this causing problems for correct programs, but it would probably lead to horribly confusing problems in incorrect code, including converting some type errors into runtime errors. A simpler approach is to say that you can not have a space after the unary "-", so (-2) parses to a numeric literal, and (- 2) parses to a function.