Re: GHC 7.10 regression when using foldr

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 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

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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

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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 wrote:

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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:

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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

Excerpts from Edward Kmett's message of 2015-01-20 15:41:13 -0800: list painstakingly

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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@serpentine.com> wrote:

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On Tue, Jan 20, 2015 at 3:02 PM, Herbert Valerio Riedel <

hvr@gnu.org>

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wrote:

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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.