Note that the "a" in the signature "IO (IOArray Int a)" is *not* the same as the one in the signature of toHeap; the scope of that type variable is the signature itself, not the following equation(s). You have in effect done the opposite of what you intended --- instead of asserting it is the same, you asserted that it is a *different* one, by handing the compiler an unexpected `a` which must be assumed to represent a distinct type.

If you need to extend the scope of a type variable like this, you need the ScopedTypeVariables extension, and to declare the type variable as having extended scope with an explicit `forall`: