Having struglled over this for the better part of a day and only becoming more frustrated the more i try to understand it, i once again seek help :) I understand how basic folds work, i.e foldr replaces (:) with some parameter and [] by another i.e foldr (+) 0 [1,2,3] becomes 1+(2+(3+0)) I also understand how to write my own fold function. What i don't understand quite is how to use them. given this data type and this fold function i wrote: data Music = Note Pitch Octave Duration | Silence Duration | PlayerPar Music Music | PlayerSeq Music Music | Tempo (Ratio Int) Music data Pitch = Cf | C | Cs type Octave = Int type Duration = Ratio Int foldMusic :: (Pitch -> Octave -> Duration -> a) -> (Duration -> a) -> (a -> a -> a) -> (a -> a -> a) -> (Ratio Int -> a -> a) -> Music -> a foldMusic n _ _ _ _ (Note pitch octive duration) = n pitch octive duration foldMusic _ s _ _ _ (Silence duration) = s duration foldMusic n s p1 p2 t (PlayerPar partOne partTwo) = p1 (foldMusic n s p1 p2 t partOne)(foldMusic n s p1 p2 t partTwo) foldMusic n s p1 p2 t (PlayerPar partA partB) = p2 (foldMusic n s p1 p2 t partA)(foldMusic n s p1 p2 t partB) foldMusic n s p1 p2 t (Tempo rate part) = t rate (foldMusic n s p1 p2 t part) I understand that when i use the foldMusic function i need to pass it 5 parameters. given the type signiature, why can i pass (+) as a parameter for p1 but not for n, what determines what can be passed as a parameter, because they all have the return type a?? I attempted to create a function that utilises the foldMusic function that counts the number of notes: count_notes :: Music -> Integer count_notes = foldMusic (\_-> \_ -> \_ -> 1) (\_ -> 0) (+) (+) (\_ -> \_ -> 0) it appears to work, i think. Yet i'm still not certain of how it does so. This confuses me, Is there anyway to represent other fold functions in a tree like representation as foldr (+) 0 would appear as such? + 1 \ + 2 \ + 3 \ 0 Regards, confused Patrick _________________________________________________________________ Hot chart ringtones and polyphonics. Go to http://ninemsn.com.au/mobilemania/default.asp