[Haskell-cafe] Stacking data types

[KC]

What about record types?


On Wed, Apr 6, 2011 at 11:58 AM, Yves Parès <limestrael at gmail.com> wrote:
> I meant:
>
> chara = Character 100 $ Armor 40 $ Gun 12 ()
>
> 2011/4/6 Yves Parès <limestrael at gmail.com>
>>
>> Hello Café,
>>
>> I'm trying to get some modular data types.
>> The idea that came to me is that I could stack them, for instance :
>>
>> data Character a = Character { life :: Int,
>>                                charaInner :: a }
>>
>> data Gun a = Gun { firepower :: Int,
>>                    gunInner :: a }
>>
>> data Armor a = Armor { resistance :: Int,
>>                        armorInner :: a }
>>
>> Then a character with a gun and an armor can be build this way:
>>
>> chara = Character 100 $ Armor 40 $ Gun 12
>>
>> The idea now is to be able to get some part of the character:
>>
>> itsGun :: Character ?? -> Gun ??
>> itsGun = content
>>
>> Then content would be a class method:
>>
>> class Has b a where
>>     content :: a -> b
>>
>> And it would be recursively defined so that:
>>
>> instance (Has c b, Has b a) => Has c a where
>>     content = (content :: b -> c) . (content :: a -> b)
>>
>> Then itsGun would be more like:
>>
>> itsGun :: (Has Gun a) => a -> Gun ??
>> itsGun = content
>>
>> But after some juggling with extensions (ScopedTypeVariables,
>> UndecidableInstances, IncoherentInstances...) I can't get it working.
>>
>> Has someone a simpler way to achieve modular types?
>
>
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-- 
--
Regards,
KC