I need help with an "ambiguous type variables" error. My problem domain is backtracking search to construct musical phrases by choosing notes that fulfill as many simultaneous criteria as possible. At the beginning, I always know that I need to choose N notes, and they will be chosen in a particular order, one at a time. I don't need to worry about cycles, therefore. There is a way of calculating a "goodness" score, given a series of the first M notes (where M ranges from 2 to N). I'm looking for the solution with the best score. Because my problem may be too large to do a complete search, I think it would be great to be able to do a combination of breadth and depth searching... sort of like a chess program that searches X moves ahead but not all the way to the end of the game. So I start with 1 note and do a complete search on all partial solutions with X notes looking for the best score (dealing with ties somehow, not sure yet how), then take the first note from the best solution, then do a complete search on all partial solutions with 2 to X+1 notes, and so forth until X=N. I have in mind many possible ways of representing phrases and "moves," so I wanted to implement my basic algorithm in a class. I need three data types: - "the data" : a representation of a solution with from 1 to N choices applied - "a choice" : a representation of a choice of note at a particular point in the phrase - "memo" : a way of tracking the best partial solution(s) found during this phase of the search Here's my code so far In the following the type variable "d" is "the data", "c" is "a choice" and "memo" is a memo. The error is Ambiguous type variables 'd0', 'c0' in the constraint: (Bt d0 c0 memo) arising from a use of 'newMemo' class Bt d c memo | d -> c, d -> memo where -- a solution state is checked against the best solutions stored in -- the memo, and possibly replaces or augments the list of best -- solutions. updateMemo :: memo -> d -> memo newMemo :: memo pickBest :: Int -> memo -> d isSolution :: d -> Bool isSolution x = stateSize x == solutionSize x -- number of choices applied so far stateSize :: d -> Int -- note that data of type 'd' includes an indication of what the -- final solution size is, something put there when d is initialized solutionSize :: d -> Int enumerateChoices :: d -> [c] -- choices available at this point in the -- construction of the state -- compute a goodness score scoreState :: d -> Double applyChoice :: d -> c -> d -- note that this is only partially implemented. Never mind that -- it's not complete or correct, I'm still trying to understand -- where my error arises. -- -- What is will eventually do: completely searches N levels deep -- before picking best score, then takes best partial solution, -- backs up N-1 levels and does another complete search, -- etc. finally displays solution with best score. limBacktrack :: Int -> d -> memo limBacktrack n d = limBacktrack' n newMemo d limBacktrack' :: Int -> memo -> d -> memo limBacktrack' nGoal memo d | stateSize d == nGoal = updateMemo memo d | otherwise = foldl g memo (enumerateChoices d) where -- g :: memo -> choice -> memo g memo choice = limBacktrack' nGoal memo (applyChoice d choice) The error is Ambiguous type variables 'd0', 'c0' in the constraint: (Bt d0 c0 memo) arising from a use of 'newMemo'