[Haskell-beginners] Intervals?

I have been trying to optimize some research I was doing in my spare time. To spare complexity, it involves music-- I have a class, defining each note in an octave, and I want to write a function that will convert between two notes and an interval-- so, for instance: Foo As {- A sharp-- can't use # -} B = 1 because in the standard scale, A# and B are one semitone apart. My question, however, boils primarily down to this: Is there a way to make Foo :: Note -> Note -> Int Foo a b = a-b work?