[Haskell-beginners] data type design question

[Alexander Dunlap]

On Thu, Jul 31, 2008 at 2:42 AM, Markus Barenhoff <alios at alios.org> wrote:
> 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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>
Hi,

Maybe you could add a "wrapper" datatype in addition to the GADT solution:

data TC
  = TCInt (T Int)
  | TCString (T String)
  | TCList a (T [T a])
  | TCDict (T DictVal)

Then your parser returns a value of type TC and you can unwrap that
using case tc of { ... } to get a value of type T. I haven't tested
this, but it seems like it ought to work.

Alex