| I eagerly awaited the Sep1999 hugs release because I hoped the MPTC
| dependencies feature would allow a certain capability I was looking
| for. I was disappointed. I think this is all best explained with an
| example:
|
| > class Zippable a b | a -> b, b -> a where
| > zip' :: a -> [b]
| >
| > instance Zippable () () where ...
| > instance (Zippable b c) => Zippable ([a],b) (a,c) where ...
| >
| > test = zip' ("abcd",())
|
| Hugs-Sep1999 gets the following error message:
|
| > ERROR "Zippable.hs" (line 14): Unresolved top-level overloading
| > *** Binding : test
| > *** Outstanding context : Zippable () b
Try adding a dummy argument to test (so the monomorphism restriction
doesn't kick in), and then look at the type that Hugs infers:
> test () = zip' ("abcd", ())
Main> :info test
test :: Zippable ([Char], ()) (Char,a) => () -> [(Char,a)]
Now you can see that Hugs has done some improvement (i.e., it has
determined that the second parameter must be a type of the form
(Char, a)), but hasn't gone all the way that you expected in
determining that a = (). In the presence of overlapping instances,
such a conclusion would not be valid.
Curiously, however, when you type in test() (or similar variant) at
the evaluator prompt, it displays a message that suggests a different
constraint here, namely Zippable () a, which really looks like it
could have been improved by using the first instance. What's happened
is that, in a desperate attempt to try and resolve overloading, Hugs
has resorted to doing some context reduction, and thereby reduced
the constraint to Zippable () a. It's easy enough to add the extra
lines that are needed to persuade Hugs to try for a further improvement,
and in this particular case, it then works as you expect. A total of
four lines are required; look for a patch soon.
The fact that this would get your program to work, however, really is
just a coincidence. There are technical issues to be resolved
here in understanding how we can get Hugs to make more use of the
information on the left of the => symbol in an instance declaration,
without requiring support for overlapping instances to be turned off.
| P.S. yes, I did remember to use -98 to enable the hugs extensions
It wouldn't have accepted the functional dependency syntax if you
hadn't!
All the best,
Mark
