Jules Bean wrote:
Peter Padawitz wrote:
Jules Bean wrote:
Peter Padawitz wrote:
Functional dependencies don't work in my case. Actually, I don't see why they should.
Ah well, it's cruel to say that without explaining to us why!
Cause I don't see why the instantiation conflicts pointed out by others would vanish then.
They would.
If it's really true that there is only one possible choice of b,c,d for any particular a, then there are no conflicts, so you'd get no errors.
How can ghci know this even if no instance has been defined?
So the fundep would solve the problem.
But, actually, it doesn't :-(
class Java (a,b,c,d) where ....
Yeah... but ghc accepts only type variables here, not arbitrary polymorphic types.
Indeed, but there is a workaround:
class Java all a b c d | all -> a, all -> b, all -> c, all -> d, a,b,c,d -> all
Same problem. If I omit the comp functions (see below), everything works. If I add them, all proposed solutions fail with error messages of the form Could not deduce (Java block1 ....) from the context (Java block ....) arising from use of `prod' at ... (see also Ben Franksen's comment from yesterday). *************** type Block = [Command] data Command = Skip | Assign String IntE | Cond BoolE Block Block | Loop BoolE Block data IntE = IntE Int | Var String | Sub IntE IntE | Sum [IntE] | Prod [IntE] data BoolE = BoolE Bool | Greater IntE IntE | Not BoolE class Java block command intE boolE where block_ :: [command] -> block skip :: command assign :: String -> intE -> command cond :: boolE -> block -> block -> command loop :: boolE -> block -> command intE_ :: Int -> intE var :: String -> intE sub :: intE -> intE -> intE sum_ :: [intE] -> intE prod :: [intE] -> intE boolE_ :: Bool -> boolE greater :: intE -> intE -> boolE not_ :: boolE -> boolE compBlock :: Block -> block compBlock = block_ . map compCommand compCommand :: Command -> command compCommand Skip = skip compCommand (Assign x e) = assign x (compIntE e) compCommand (Cond be cs cs') = cond (compBoolE be) (compBlock cs) (compBlock cs') compCommand (Loop be cs) = loop (compBoolE be) (compBlock cs) compIntE :: IntE -> intE compIntE (IntE i) = intE_ i compIntE (Var x) = var x compIntE (Sub e e') = sub (compIntE e) (compIntE e') compIntE (Sum es) = sum_ (map compIntE es) compIntE (Prod es) = prod (map compIntE es) compBoolE :: BoolE -> boolE compBoolE (BoolE b) = boolE_ b compBoolE (Greater e e') = greater (compIntE e) (compIntE e') compBoolE (Not be) = not_ (compBoolE be)