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
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