Isn't that what we have here? a function of type (a -> A -> B) cannot use the first argument in any meaningful way.
I think, he wants to document that the type variable 'a' is not used in the *type*.
Yeah, that's the idea, sorry if I wasn't clear. In the case of const, I might write const :: a -> _b -> a To document that 'b' intentionally appears only once, but this is only my convention and I've never seen anyone else use it. In the case of 'const' it's pretty obvious and unnecessary, but in a longer signature it might help a bit, especially if you are using phantom types and some functions intentionally ignore type arguments. I haven't used them before, but in the presence of scoped type variables, wouldn't have a reader have to go look for internal definitions to reassure himself that the type is in fact ignored entirely? It's not a big issue, but it seemed like a nice symmetry with pattern matching syntax.