On Sat, Mar 19, 2011 at 2:00 AM, Patrick LeBoutillier wrote:

Hi,

I'm writing a music library and have the following types and functions:

data Note = C | D | E | F | G | A | B | Sharp Note | Flat Note raise C = Sharp C

data Quality = Major | Minor

data Chord = Chord Quality Note  -- Note is the root data Scale = Scale Quality Note  -- Note is the tonic

What I would like to do is to be able to lift the raise function and apply it to Chords and Scales. Something like this:

fmap raise (Chord Major C) = Chord Major (Sharp C)

But these are not functors as it doesn't seem to make sense for them to be polymorphic. A chord can't really made of anything except Notes.

You can write your own plain old functions: mapChord :: (Note -> Note) -> Chord -> Chord mapChord f (Chord q n) = Chord q (f n) mapScale :: (Note -> Note) -> Scale -> Scale mapScale f (Scale q n) = Scale q (f n) You can also create your own type class, if that's convenient for you class NoteMap a where noteMap :: (Note -> Note) -> a -> a instance NoteMap Chord where noteMap = mapChord instance NoteMap Scale where noteMap = mapScale HTH, =) -- Felipe.