> The *only* way I have been able to make this work, after lots of
> trying and mind-bending, is to introduce a "phantom" type to allow me
> to combine things appropriately:
>
> > data MkFinMap m k a = MkFinMap (m (Pair k a))
>
> > instance (SortedList m (Pair k a), ZeroVal a) =>
> > FiniteMap (MkFinMap m) k a where
> > emptyMap = MkFinMap emptySort
> > bindMap (MkFinMap t) k x = MkFinMap (addToSort t (Pair k x))
> > lookupMap (MkFinMap t) k =
> > let r = findInSort t (Pair k zer)
> > in case r of
> > Nothing -> Nothing
> > Just (Pair k' x) -> Just x
I don't think you can avoid this. You have two type constructors:
class SortedList s a where ...
class FiniteMap m k a where ...
s has kind *->*
m has kind *->*->*
You want to say
instance SortedList s (Pair k a) => FiniteMap m k a where...
but there's a relationship between s and m, namely
m k a = s (Pair k a)
That is the relationship your MkFinMap states.
I don't know how to improve on this... except to give sortedList a
key and a value.
You don't need class ZeroVal.. just use 'undefined' instead.
Simon
