The other much simpler solution no one has mentioned yet is to just pull 'subsome' out as its own top-level declaration. Having such a big function nested locally within a 'let' is ugly anyway, and it makes it harder to test and debug than necessary. -Brent On Tue, Jan 03, 2012 at 05:44:01PM +0100, Yves Parès wrote:
Remove subsome type signature. You are redeclaring type variables which obviously cannot match those of legSome. This cannot work without scoped type variables (and ad-hoc foralls to bring them to scope, of course).
2012/1/3 Yucheng Zhang
As I investigated the code more carefully, I found that the type unification failure may not be related to the suspected class constraint on data constructor.
I have made minor changes to the original code to remove the Ord constraint, including introducing a FakedMap with no requirement on Ord. The type unification failure continues:
Couldn't match type `nt1' with `nt' `nt1' is a rigid type variable bound by the type signature for subsome :: [RRule nt1 t1 s1] -> Either String ([t1], s1) at xx.hs:34:19 `nt' is a rigid type variable bound by the type signature for legSome :: LegGram nt t s -> nt -> Either String ([t], s) at xx.hs:29:1 Expected type: [Symbols nt1 t1] Actual type: [Symbols nt t] In the first argument of `makeWord', namely `r' In the expression: makeWord r
The complete changed code follows:
data Symbols nt t = NT nt -- ^ non terminal | T t -- ^ terminal deriving (Eq, Ord)
type Sem s = [s]->s
data Rule nt t s = Rule { refined :: nt , expression :: [Symbols nt t] , emit :: Sem s }
type RRule nt t s = ([Symbols nt t], Sem s)
data FakedMap a b = FakedMap
delete :: k -> FakedMap k a -> FakedMap k a delete a b = b
lookup :: k -> FakedMap k a -> Maybe a lookup a b = Nothing
data LegGram nt t s = LegGram (FakedMap nt [RRule nt t s])
legSome :: LegGram nt t s -> nt -> Either String ([t], s) legSome (LegGram g) ntV = case Main.lookup ntV g of Nothing -> Left "No word accepted!" Just l -> let sg = legSome (LegGram (Main.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
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