"Boespflug, Mathieu"
Hi Ben,
could we enlarge the options a bit? I feel that we're in a false dichotomy currently. I think the issue isn't so much what warnings you see from the compiler with common settings, so much as "what warnings will cause my build to fail?". i.e. the issue isn't what's in -Wall, it's what goes in -Werror...
I can certainly see this point. As pointed out by Herbert, this is best provided by #11219 which unfortunately didn't make the 8.0 release.
If we don't include -Wcompat in -Wall, then I suspect Ben is quite right that many will ignore / not be aware of -Wcompat hence rendering the 3 release policy rather moot (it's much less useful if developers don't /know/ they're incompatible until the last minute). If we do include it then packages with -Wall -Werror will fail far too early: i.e. way before the compat change actually happens, 3 compiler releases down the line. So may I suggest the following:
* include -Wcompat and -Wall, but make it "magic" so that -Wcompat never causes a build to fail even when users set -Wall -Werror.
This should address the concerns I've been hearing by folks at large companies who say that at their company a warning is equivalent to an error, because they always compile with -Wall -Werror. Otherwise warnings just accumulate, hence no one pays attention to them, hence warnings become useless.
As an aside: I believe -Wall should do what it says on the tin: enable all warnings.
I am sympathetic to this argument. For better or for worse, however, there seems to be precedent for excluding some warnings in -Wall. Clang, for instance, uses -Weverything for "give me everything". Up until this point we have followed this model (although don't have a -Weverything); for instance, we disable -Wimplicit-prelude in -Wall since it is inappropriate for most users. I'm not sure whether we should break with this tradition as users are quite used to invoking their compiler with `-Wall` during development. Throwing warnings about implicit Prelude imports at them would be surprising to say the least.
When GHC comes up with new warnings, we should pretty much never hesitate to add them to the -Wall set.
I agree. This is why Simon wrote an explicit declaration that -Wall is not stable to the list a few weeks ago. We really don't want to worry about breaking builds of (potentially broken) user code when adding warnings. Cheers, - Ben