GHC 7.10 regression when using foldr

[Edward Kmett]

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.
> > > > >
> > >
>
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