[Haskell-cafe] Stacking data types

[Job Vranish]

I think you want something like this:

{-# Language MultiParamTypeClasses
           , FlexibleInstances
           , FunctionalDependencies
           , UndecidableInstances
           , FlexibleContexts
           , OverlappingInstances

           #-}
data Character a = Character { life :: Int,
                               charaInner :: a }
                  deriving (Show)

data Gun a = Gun { firepower :: Int,
                   gunInner :: a }
             deriving (Show)

data Armor a = Armor { resistance :: Int,
                       armorInner :: a }
               deriving (Show)


class HasInner f where
  getInner :: f a -> a

instance HasInner Character where
  getInner = charaInner

instance HasInner Gun where
  getInner = gunInner

instance HasInner Armor where
  getInner = armorInner


class Has b a | a -> b where
    content :: a -> b

instance (Has b a, HasInner f) => Has b (f a) where
    content a = content $ getInner a

instance (HasInner f) => Has a (f a) where
    content a = getInner a

chara = Character 100 $ Armor 40 $ Gun 12 ()

itsGun :: (Has (Gun b) a) => a -> Gun b
itsGun = content

You were missing a mechanism to extract the inner value from your datatypes.

- Job


On Wed, Apr 6, 2011 at 2:57 PM, Yves Parès <limestrael at gmail.com> wrote:

> 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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