Hello all,
I'm trying to build a variation on Maps which supports a fast
concat-like operation, for a library I'm writing. I'd rather not
re-implement Data.Map, so I'm having a try with GADTs.
The relevant part of my source file:
-
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
-
Unfortunately, this doesn't work:
Data/MetaMap.hs:45:20:
Could not deduce (Ord k) from the context (Monad m)
arising from use of `Map.lookup' at Data/MetaMap.hs:45:20-33
Possible fix: add (Ord k) to the type signature(s) for `lookup'
In the expression: Map.lookup k m
In the definition of `lookup': lookup k (Map m) = Map.lookup k m
This error happens because the Ord constraint isn't propagated into the
function body. Of course, I can't add Ord k to the type signature for
lookup, because one can't deduce Ord k from Ord (k1,k2).
Is there a clean way around this error?
Thanks for your time,
Mike Hamburg
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