On Tue, Jan 3, 2012 at 5:43 PM, AUGER Cédric
legSomeb :: LegGram nt t s -> nt -> Either String ([t], s) -- but without it I have an error reported legSomeb (LegGram g) ntV = case M.lookup ntV g of Nothing -> Left "No word accepted!" Just l -> let sg = legSomeb (LegGram (M.delete ntV g)) subsome :: [RRule nt t s] -> Either String ([t], s) subsome [] = Left "No word accepted!" subsome ((r,sem):l) = let makeWord [] = Right ([],[]) makeWord ((NT nnt):ll) = do (m, ss) <- sg nnt (mm, sss) <- makeWord ll return (m++mm, ss:sss) makeWord ((T tt):ll) = do (mm, sss) <- makeWord ll return (tt:mm, sss) in case makeWord r of Right (ll, mm) -> Right (ll, sem mm) Left err -> subsome l in subsome l
I found it compiling well if removing this line: subsome :: [RRule nt t s] -> Either String ([t], s) It seems to me that the compiler is not sure the two 'nt' are equal. The ScopedTypeVariables can make the compiler believe they are equal.