apfelmus wrote:
> {-# OPTIONS_GHC -fglasgow-exts -#}
> import Prelude hiding (lookup)
>
> class Map map key a | map key -> a where
> lookup :: key -> map -> Maybe a
> adjust :: (a -> a) -> key -> map -> map
>
> instance (Map m k m', Map m' k' a) => Map m (k,k') a where
> lookup (k,k') m = lookup k m >>= lookup k'
> adjust f (k,k') m = adjust (adjust f k') k m
Uh, that actually needs -fundecidable-instances or something similar.
Using explicit functor composition doesn't resolve the problem
class Map map key | map -> key where
lookup :: key -> map a -> Maybe a
adjust :: (a -> a) -> key -> map a -> map a
data O f g a = O (f (g a))
instance (Map m k, Map m' k') => Map (m `O` m') (k,k') a where
...
-- but how to make ((m `O` m') a) a map for (m' a)?
With associated type synonyms, this program can't even be formulated.
Any ideas?
For the time being, you can specialize the key-pairing for concrete maps
in question.
newtype UniverseF a = UniverseF [a]
newtype GalaxyF a = GalaxyF [a]
type Universe = UniverseF Galaxy
type Galaxy = GalaxyF Planet
data Planet = ...
instance Map (UniverseF a) k a where ...
instance Map m k a => Map (UniverseF m) (Int,k) a where ...
Oh, and you need a dummy instance for Planets as "singleton"-maps.
instance Map Planet () Planet where ...
Regards,
apfelmus
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
