Export lists in modules
Benjamin Franksen
benjamin.franksen at bessy.de
Fri Mar 3 05:17:22 EST 2006
On Friday 03 March 2006 10:52, Malcolm Wallace wrote:
> Marcin 'Qrczak' Kowalczyk <qrczak at knm.org.pl> writes:
> > > But if contexts-on-datatypes worked correctly,
> > > data Set a = Ord a => ....
> > > then even the "real" map from Data.Set:
> > > map :: (Ord a, Ord b) => (a -> b) -> Set a -> Set b
> > > could be an instance method of Functor.
> >
> > fmap ($0) . fmap const :: Functor f => f a -> f a
> >
> > When applied to Set Int, how would it represent the intermediate
> > set of functions? Or if it was disallowed, on what basis?
>
> Clever example. The intermediate set is :: Set (Int->b->Int), which
> does not satisfy the construction constraint (Ord a) => Set a. But
> persuading the type system to reject this is tricky I agree. I'm not
> a type hacker, so I don't know how to go about it. I suppose if the
> complete set of class instances for the entire program were known,
> then you could have a negation predicate, asserting that the
> intermediate type (Int->b->Int) does not have an instance of Ord, and
> somehow record this in the type environment
>
> fmap ($0) . fmap const :: (Functor f, not Ord a) => f a -> f a
>
> But then, there are a whole host of classes that (->) is not an
> instance of, so this is pretty useless because the constraints will
> grow enormously for little benefit.
>
> In short, I suppose the reason contexts-on-datatypes are as they are
> currently, is because no-one yet knows how to solve your example.
Well, that is what wft constraints are for, see
http://www.cs.chalmers.se/~rjmh/Papers/restricted-datatypes.ps
where the above problem is discussed and a solution proposed.
(hasn't this been discussed here, lately?)
Ben
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