[Haskell-beginners] data type design question

[Bas van Dijk]

On Wed, Jul 30, 2008 at 11:09 AM, Markus Barenhoff <alios at alios.org> wrote:
> Hi,
> I have written a parsec parser for reading a marshallaled dataformat.
>
> The returned data structure is based on the following data type:
>
>> data T = TString String
>>        | TInt Integer
>>        | TList [TorrentT]
>>        | TDict [(TorrentT, TorrentT)]
>
> I think TString and TInt are clear. The elements of a TList always have
> the same "type" (same constructor). The TDict is a dictionary where the
> key is always a TString but the value can be of any of the other "types",
> even in the same dictionary. F.e the key "foo" may map to a TInt while the
> key "bar" maps to another TDict.
>
> I'am not happy with this declaration, but I'am not sure how to express this
> better.
>
> One haskell data type for each of the four and then using type classes?
>
> Maybe something like this? :
>
>> type TString = String
>> type TInt = Integer
>> type TList = TC t => [t]
>> type TDict = (TC t) => [(TString, t)]
>
>
>> class TC where ...
>
>> instance TC TString
>> instance TC TInt
>> instance TC TList
>> instance TC TDict
>
>
> Thnx for some inseperation!
> Markus
>
> --
> Markus Barenhoff - Münster - Germany - Europe - Earth
> e-mail: alios at alios.org - jabber: alios at jabber.ccc.de - icq: 27998346
>
>
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>

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

-------------------------------------------------------
{-# 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)]
-------------------------------------------------------

Note that it isn't possible to create a 'TList [n,n,n,s]' for example.


I don't have much time to explain GADTs and ExistentialQuantification
but you can read about them in the GHC user guide:

http://www.haskell.org/ghc/docs/6.8.3/html/users_guide/index.html

good luck,

Bas