I think I should suggest HList from Oleg Kiseliov.
http://hackage.haskell.org/package/HList
That way you will have something along those lines:
-- fields descriptors:
data Character
data Gun
data Armor
data Life
-- values for fields:
data Vulcan = Vulcan { vulcanAmmoCount :: Int}
data Player = Player { playerName :: String }
player = (Character, Player) :*: (Gun,Vulcan) :*: (Armor,50) :*: HNil
HList has all the machinery to create records, get a field from record
(something like "getField rec Character"), test for field in record
(Maybe value), etc.
2011/4/6 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?
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe