It's actually simpler in some ways, because we know that "a" must have arity 1. What we know is that "a" should be a function which takes a "b", that "a b" should be a function which takes a "c", and "a b c" should take a "d".

As a practical consideration, this rarely if ever becomes an issue, and if it does, the type checker will tell you straight away.

Type annotations can make clear what isn't intuitively clear with a function's signature, and since the correctness of the type checker is rigorously proven, I don't see anything particularly wrong with "relying" on the type checker.