[Haskell-cafe] Solved but strange error in type inference

[Yucheng Zhang]

On Tue, Jan 3, 2012 at 5:43 PM, AUGER Cédric <sedrikov at gmail.com> wrote:
> 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.