ghc warnings messed up in 6.01

[John Meacham]

has anyone else noticed that the warnings generated by ghc appear to
have become quite incorrect in many cases?

in particular, the 'defined but not used' warning is generated
spuriously a lot. almost as if it is being checked after dead code
elimination and desugaring of some sort..

here is an example which touches all the reproducable bugs I have found.

> module Main(main) where
> import List(nub)
> import Monad()
> 
> as = repeat 'a'
> bs = repeat 'b'
> ds = nub as
> 
> cs = [(a,b) | a <- as | b <- bs]
> 
> main = print $ take 10 cs        


;ghc -W -fglasgow-exts  Foo.hs

Foo.hs:1:
    Warning: Module `List' is imported, but nothing from it is used
             (except perhaps instances visible in `List')

Foo.hs:1:
    Warning: Module `Monad' is imported, but nothing from it is used
             (except perhaps instances visible in `Monad')

Foo.hs:7: Warning: Defined but not used: ds

Foo.hs:9: Warning: Defined but not used: b

Foo.hs:9: Warning: Defined but not used: a


notice:

1) it claims nothing is used by List, when 'nub' definatly is being
used. (technically true, since ds is not used, but this warning is more
confusing than anything. my first inclination is to delete the import
line, causing the program to fail to compile, the only fix seems to be
to track down the root by hand)

2) it claims Monad is unused, when I specifically imported it with ()
meaning I don't want any names from it. (i.e., I just want instances)

3) defined but not used 'ds': yay! the only valid warning.

4,5) variables bound in parallel list comprehensions always seem to
generate warnings.  


AFAIK, none of these were problems in earlier versions of ghc, they are
quite anoying as they obscure valid warnings. my guess is some
desugarings and warning passes were accidentally switched around..
problem 1 is particularly bad, since a single unused toplevel could
cascade to generating a ton of warnings, all of which must be sorted
through to find the actual cause. the others are just anoying as there
doesn't seem to be a workaround short of disabling warnings.

        John
-- 
---------------------------------------------------------------------------
John Meacham - California Institute of Technology, Alum. - john at foo.net
---------------------------------------------------------------------------