> A more concrete way to formulate a problem that I believe to be > equivalent is this. Implement the following interface > > module TypedFM where > data FM k -- Abstract; finite map indexed by keys > of type k > data Key k a -- Abstract; a key of type k, indexing a > value of type a > > empty :: FM k > insert :: Ord k => FM k -> k -> a -> (FM k, Key k a) > lookup :: Ord k => FM k -> Key k a -> Maybe a > > The point is that the keys are typed, like Refs are. But the finite map > FM is only parameterised on k, not a, so it can contain (key,value) > pairs of many different types. > > I don't think this can be implemented in Haskell, even with > existentials. But the interface above is completely well typed, and can > be used to implement lots of things. What I *don't* like about it is > that it embodies the finite-map implementation, and there are too many > different kinds of finite maps.
Here is a Haskell 98 implementation: > module TypedFM > where > data FM k = FM > data Key k a = Key k a > empty = FM > insert FM k a = (FM, Key k a) > lookup FM (Key k a) = Just a Cheers, Ralf _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
