[Haskell-cafe] Can anybody give me some advice on this?

[Georgel Calin]

Hello everybody,

I have a piece of code that gives me headaches for some time now.

Simply put, I would like to know which is the best way to overpass a
"Couldn't match expected type * against inferred type *"-error and an
"Occurs check: cannot construct the infinite type:"-error in the following
situation:

{-# OPTIONS -fglasgow-exts #-}
> module Simple where
> import Text.ParserCombinators.Parsec
>
> data HData a = O | C a deriving (Eq,Ord,Show)
> data IN l = IN Int (HData l) deriving (Eq,Ord,Show)
> data CH l = CH Char (HData l) deriving (Eq,Ord,Show)
> -- data type is well-defined:
> sample = C(IN 0 (C(CH 'a' (C(IN 1 (C(CH 'b' (C(IN 2 O)))))))))
>
> embeddedParser types =  do string "end"; spaces; return O
> {-
>                     <|> do let h = head types
>                               let t = tail types
>                               case h of
>                                  1 -> do aux <- pInt
>                                             rest <- embeddedParser $t++[h]
>                                             return $ C (IN aux rest)
>                                  2 -> do aux <- pCh
>                                             rest <- embeddedParser $t++[h]
>                                             return $ C (CH aux rest)
>                                  _ -> error "unallowed type"
> -}
> pInt =  do n <- fmap read $ many1 digit; return $ fromInteger n
> pCh =  do c <- letter; return $ c
> simple = embeddedParser [1,2]
>
> -- the above result from sample I would like to get by running
> -- parseTest simple "0a1b2end"
>

The way I see it, the defined datatype works but I am a bit clueless about
how to modify the parser to accept things of the type (e.g.): HData (IN (CH
(IN (CH (IN a))))) (and in general of any finite type embedded like this).

Thanks in advance for your help,
George
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