6 May
2007
6 May
'07
10:05 a.m.
On Sunday 06 May 2007, Mike Hamburg wrote:
data MetaMap k v where Map :: Ord k => Map.Map k v -> MetaMap k v Cat :: MetaMap k1 (MetaMap k2 v) -> MetaMap (k1,k2) v {- other, similar constructors -}
lookup :: Monad m => k -> MetaMap k v -> m v lookup k (Map m) = Map.lookup k m lookup k (Cat m) = case k of (k1,k2) -> lookup k1 m >>= lookup k2
What version of ghc are you using? The snippet works here, but I believe that GADTs only store the Ord dictionary as is needed here in HEAD. It won't work if you're trying to do this with 6.6. -- Dan Doel