This also doesn't seem like it would work very well with making an instance for IntMap. I guess I can't have everything.
It is not that difficult to make instances for IntMap:
data TypeCast k IM.Key => WIM k e = WIM (IM.IntMap e) deriving Show
instance Collection (WIM IM.Key e) (IM.Key,e) where empty = WIM $ IM.empty fromList = WIM . IM.fromList size (WIM x) = IM.size x
instance Mapping WIM IM.Key e where lookup k (WIM im) = maybe (fail "not found") return (IM.lookup k im)
-- verbatim from the HList paper class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x
What do you suggest that I do for map? What sort of class should it be in? Functor. The similarity of the names `map' and `Mapping' is confusing;
It seems that restricted data type WIM k e is better than GADT approach shown yesterday. We define `WIM k e' in such a way that the type parameter 'k' is restricted to satisfy a particular constraint, being equal to IM.Key in our case. If the compiler cannot see that this is the case, the compiler will raise an error. For example, one may try to write instance Collection (WIM k e) (k,e) where ... and see what happens. So, the constraint enforcement in our case happens statically and quite early. the two describe different concepts in the framework at hand.