incomplete overloading resolution

[Ralf Laemmel]

Dear Hugs-Bugsers,

what a funny bug ...


import Monad

class Monad m => C1 m x
class (Monad m, C1 m x) => C2 m x
  where
    c2 :: x -> m x

instance Monad m => C1 m Bool
instance C2 Maybe Bool
  where
    c2 = return


test :: Maybe Bool
test = c2 True


1. If you evaluate "test",
   then you get as expected "Just True".

2. Now remove the "Monad M" constraint
   in the class C2. This is of course
   legal and semantics-preserving since
   the monad constraints is implied by C1
   anyway.

3. Now evaluate "test" again.
   Oops, it diverges.

I did this Hugs Version February 2001 under Solaris
and Linux. Command line option -98 is needed of course.
Funny enough, if CTRL-C the evaluation of "test"
and you try it the second time (without reloading
anything etc.), then you see "Program error: {_Gc Black
Hole}". Of course, there is no such black hole.

I extracted the above fragment from a huge problem.
The scheme is the following. It seems have to do 
with multi-parameter classes. It definitely has to
do with multi-layered class hierarchies where one
class has a class-wide superclass, as C2 has C1 in
the example. It seems that the superclass is
properly propagated during type-inference/checking
but it is not properly propagated, as for as code
determination in overloading resolution.

Please, let me know if I can be of further assistance.
I am actually amazed how general this scheme is
(3 classes, 2 parameters). How does it come that it
was not detected much earlier.

Anyway, good luck,
Ralf


P.S.: I have more problems in a larger application.
Certain overloaded expressions diverge. I can recover
from that usually by using explicit function types
for top-level functions, where again -- as in C1 above
-- I repeat some constraints which are derivable. I was
not able to extract a simple program for that problem.
But I would be glad to help you by checking if the
problem is gone after you did the bug fix.


-- 
Dr.-Ing. Ralf Laemmel
CWI & VU, Amsterdam, The Netherlands
http://www.cwi.nl/~ralf/
http://www.cs.vu.nl/~ralf/