GHC 7.10 regression when using foldr

[Edward Z. Yang]

I don't see why that would be the case: we haven't *excluded* any
old import lists, so -ddump-minimal-imports could still
take advantage of Prelude in a warning-free way.

Edward

Excerpts from Edward Kmett's message of 2015-01-20 16:36:53 -0800:
> It isn't without a cost. On the down-side, the results of
> -ddump-minimal-imports would be er.. less minimal.
> 
> On Tue, Jan 20, 2015 at 6:47 PM, Edward Z. Yang <ezyang at mit.edu> wrote:
> 
> > I like this proposal: if you're explicit about an import that
> > would otherwise be implicit by Prelude, you shouldn't get a
> > warning for it. If it is not already the case, we also need to
> > make sure the implicit Prelude import never causes "unused import"
> > errors.
> >
> > Edward
> >
> > Excerpts from Edward Kmett's message of 2015-01-20 15:41:13 -0800:
> > > Sure.
> > >
> > > Adding it to the CHANGELOG makes a lot of sense. I first found out about
> > it
> > > only a few weeks ago when Herbert mentioned it in passing.
> > >
> > > Of course, the geek in me definitely prefers technical fixes to human
> > ones.
> > > Humans are messy. =)
> > >
> > > I'd be curious how much of the current suite of warnings could be fixed
> > > just by switching the implicit Prelude import to the end of the import
> > list
> > > inside GHC.
> > >
> > > Now that Herbert has all of his crazy tooling to build stuff with 7.10
> > and
> > > with HEAD, it might be worth trying out such a change to see how much it
> > > reduces the warning volume and if it somehow manages to introduce any new
> > > warnings.
> > >
> > > I hesitate to make such a proposal this late in the release candidate
> > game,
> > > but if it worked it'd be pretty damn compelling.
> > >
> > > -Edward
> > >
> > > On Tue, Jan 20, 2015 at 6:27 PM, Edward Z. Yang <ezyang at mit.edu> wrote:
> > >
> > > > Hello Edward,
> > > >
> > > > Shouldn't we publicize this trick? Perhaps in the changelog?
> > > >
> > > > Edward
> > > >
> > > > Excerpts from Edward Kmett's message of 2015-01-20 15:22:57 -0800:
> > > > > Building -Wall clean across this change-over has a big of a trick to
> > it.
> > > > >
> > > > > The easiest way I know of when folks already had lots of
> > > > >
> > > > > import Data.Foldable
> > > > > import Data.Traversable
> > > > >
> > > > > stuff
> > > > >
> > > > > is to just add
> > > > >
> > > > > import Prelude
> > > > >
> > > > > explicitly to the bottom of your import list rather than
> > painstakingly
> > > > > exclude the imports with CPP.
> > > > >
> > > > > This has the benefit of not needing a bunch of CPP to manage what
> > names
> > > > > come from where.
> > > > >
> > > > > Why? GHC checks that the imports provide something 'new' that is
> > used by
> > > > > the module in a top-down fashion, and you are almost suredly using
> > > > > something from Prelude that didn't come from one of the modules
> > above.
> > > > >
> > > > > On the other hand the implicit import of Prelude effectively would
> > come
> > > > > first in the list.
> > > > >
> > > > > It is a dirty trick, but it does neatly side-step this problem for
> > folks
> > > > in
> > > > > your situation.
> > > > >
> > > > > -Edward
> > > > >
> > > > > On Tue, Jan 20, 2015 at 6:12 PM, Bryan O'Sullivan <
> > bos at serpentine.com>
> > > > > wrote:
> > > > >
> > > > > >
> > > > > > On Tue, Jan 20, 2015 at 3:02 PM, Herbert Valerio Riedel <
> > hvr at gnu.org>
> > > > > > wrote:
> > > > > >
> > > > > >> I'm a bit confused, several past attoparsec versions seem to build
> > > > just
> > > > > >> fine with GHC 7.10:
> > > > > >>
> > > > > >>   https://ghc.haskell.org/~hvr/buildreports/attoparsec.html
> > > > > >>
> > > > > >> were there hidden breakages not resulting in compile errors?
> > > > > >> Or are the fixes you mention about restoring -Wall hygiene?
> > > > > >>
> > > > > >
> > > > > > I build with -Wall -Werror, and also have to maintain the test and
> > > > > > benchmark suites.
> > > > > >
> > > >
> >