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
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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