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 <yczhan...@gmail.com>
> 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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