[Haskell-beginners] data type design question

[Markus Barenhoff]

On Thu, Jul 31, 2008 at 08:58:08AM +0200, Bas van Dijk wrote:

Hi,

> Not really a beginners type answer because I need two big language
> extensions, but anyway:
>

my feeling that there is a sollution somewhere in the extenstions seems to
have been right.

> -------------------------------------------------------
> {-# LANGUAGE GADTs #-}
> {-# LANGUAGE ExistentialQuantification #-}
> 
> data DictVal = forall a. D (T a)
> 
> data T a where
>   TInt    :: Int    -> T Int
>   TString :: String -> T String
>   TList   :: [T a]  -> T [T a]
>   TDict   :: [(String, DictVal)] -> T DictVal
> 
> -- For example
> 
> n = TInt 3
> s = TString "abc"
> l = TList [n,n,n]
> d = TDict [("n", D n), ("s", D s), ("l", D l)]
> -------------------------------------------------------

This is what I was looking for. But it looks like that the problem with
the type of the "toplevel" parser still exists. The compiler wants a type:

tParser :: GenParser Char st (T a)
tParser =
    do stringParser <|> integerParser <|> listParser <|> dictParser

this causes the compiler to generate the following error:

Couldn't match expected type `[Char]' against inferred type `Int'
	When generalising the type(s) for `torrentParser'

Any further ideas?

Thanx for all you answers btw.!

Markus

-- 
Markus Barenhoff - Germany - Europe - Earth
e-mail: alios at alios.org - jabber: alios at jabber.ccc.de - icq: 27998346

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