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