On Wed, Jan 13, 2016 at 7:43 AM, Simon Peyton Jones <simo...@microsoft.com> wrote:
> An implication is that GHC is free to introduce new warnings X into -Wall. > Indeed doing so would be good, because the warning X might later move into > the default set. Indeed for such warnings, adding a "PS: this warning will > become the default in GHC 9.2" might be a useful way to signal the upcoming > change. Then you can use -Wall and look for any "PS" indicators. Yep. In general I think we don't know how _much_ noise a warning will create until it makes it into the wild, so just as with other new features its good to give them a bit of a "dry run" before deciding that they come "by default." > You don’t give a rationale for (2) but I think you intend that if someone > wants to add -Wno-X when GHC introduces X in 9.0, you don't want GHC 8.6 to > fall over. Worth articulating the rationale. Yes, that's exactly the rationale. It doesn't help us short term, but longer term it should let users fiddle with warning flags more freely. I think the general issue with three releases is not whether or not GHC introduces warnings and at what pace, but that certain _types_ of warnings (in particular redundancies, be they constraints, imports, etc) will fire on entirely desirable code due to certain migration paths. Most of the tricks we developed for backwards-compatible migrations essentially depend on certain redundancies in code for a period. Those can't be removed without hurting backwards-compatibility of code, but their presence also induces warnings. So as a whole "warning freeness" and "backwards compatible migrations" become increasingly at odds with one another. A full refactor of our warning sets would probably help in this regard, so that the default advice could be "good code is -Wlint clean but not necessarily -Wpedantic clean". Or even "is clean under -Wpedantic -Wno-redundancies". --Gershom > > | -----Original Message----- > | From: Glasgow-haskell-users [mailto:glasgow-haskell-users- > | boun...@haskell.org] On Behalf Of Gershom B > | Sent: 13 January 2016 02:20 > | To: GHC users <glasgow-haskell-users@haskell.org>; ghc-d...@haskell.org; > | Edward Kmett <ekm...@gmail.com>; Herbert Valerio Riedel <h...@gnu.org>; > Simon > | Marlow <marlo...@gmail.com> > | Subject: Re: Warnings, -Wall, and versioning policy > | > | Hi Simon. I think you raise important issues here, although I believe you’re > | mistaken in one regard. Hackage rejects -Werror but I don’t think it rejects > | -Wall. > | > | What I’d suggest is perhaps the following. > | > | 1) The libraries committee put forward -Wall cleanliness as an _aspirational > | goal_ rather than a final product, noting that the actual cleanliness might > | be with regards to `-Wall -Wno-foo -Wno-bar``. > | > | 2) GHC _change its code_ so that `ghc -Wno-wat` yields a _warning_ rather > | than an _error_ on `-W` followed by an unrecognized string. > | > | 3) No warning flags be introduced into the _default_ set without at least a > | few releases in some other set such as `-Wall`. > | > | We may also want to try to maintain a “best practices” example cabal file > | that shows how one can build with additional warnings under a “dev” flag, > and > | with fewer warnings otherwise — so that the noise inflicted on package devs > | under their builds doesn’t get inflicted on all end users, and even perhaps > | with different warning flags per ghc version flag. > | > | I think this will respect the concerns of people that like to use `-Wall`, > | want to have relatively warning clean code, and want to have some degree of > | backwards compatibility (which is not an unreasonable combination in my > | opinion). > | > | Some related > | discussion: https://ghc.haskell.org/trac/ghc/ticket/11370 and > https://ghc.has > | kell.org/trac/ghc/wiki/Design/Warnings > | > | Cheers, > | Gershom > | > | > | On January 12, 2016 at 11:18:57 AM, Simon Marlow (marlo...@gmail.com) wrote: > | > Hi folks, > | > > | > We haven't described what guarantees GHC provides with respect to -Wall > | > behaviour across versions, and as a result there are some differing > | > expectations around this. It came up in this weeks' GHC meeting, so we > | > thought it would be a good idea to state the policy explicitly. Here it > is: > | > > | > We guarantee that code that compiles with no warnings with -Wall > | > ("Wall-clean") and a particular GHC version, on a particular > | > platform, will be Wall-clean with future minor releases of the same > | > major GHC version on the same platform. > | > > | > (we plan to put this text in the User's Guide for future releases) > | > > | > There are no other guarantees. In particular: > | > - In a new major release, GHC may introduce new warnings into -Wall, > | > and/or change the meaning of existing warnings such that they trigger > | > (or not) under different conditions. > | > - GHC may generate different warnings on different platforms. (examples > | > of this are -fwarn-overflowed-literals and > | > -fwarn-unsupported-calling-conventions) > | > > | > Some rationale: > | > - We consider any change to the language that GHC accepts to be a > | > potentially code-breaking change, and subject to careful scrutiny. To > | > extend this to warnings would be a *lot* of work, and would make it > | > really difficult to introduce new warnings and improve the existing ones. > | > - Warnings can be based on analyses that can change in accuracy over > | > time. The -fwarn-unused-imports warning has changed a lot in what it > | > rejects, for example. > | > - We often introduce new warnings that naturally belong in -Wall. If > | > -Wall was required to be a fixed point, we would have to start > | > introducing new flags, and versioning, etc. and even keep the old > | > implementation of warnings when they change. It would get really messy. > | > > | > There are some consequences to this. -Wall -Werror is useful for > | > keeping your code warning-clean when developing, but shipping code with > | > these options turned on in the build system is asking for trouble when > | > building your code with different GHC versions and platforms. Keep > | > those options for development only. Hackage already rejects packages > | > that include -Wall for this reason. > | > > | > One reason we're raising this now is that it causes problems for the > | > 3-release policy > | > > | > (https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fprime.haske > | ll.org%2fwiki%2fLibraries%2f3-Release- > | > Policy&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c5d13dce17e0b47a80263 > | > 08d31bc02832%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=KzdY%2bG8jP8ofztNpN > | khKGyB5PVW1XUWbw2lCQqdSNmc%3d) which > | > requires that it be possible to write code that is Wall-clean with 3 > | > major releases of GHC. GHC itself doesn't guarantee this, so it might > | > be hard for the core libraries committee to provide this guarantee. I > | > suggest this requirement be dropped from the policy. > | > > | > Cheers, > | > Simon > | > _______________________________________________ > | > Glasgow-haskell-users mailing list > | > Glasgow-haskell-users@haskell.org > | > > | > https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.haskell. > | org%2fcgi-bin%2fmailman%2flistinfo%2fglasgow-haskell- > | > users&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c5d13dce17e0b47a802630 > | > 8d31bc02832%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=f395AsIaHpKb8S9z4CAo > | qfGhiDxa5tzQUo8Sm5%2bgKPQ%3d > | > > | > | _______________________________________________ > | Glasgow-haskell-users mailing list > | Glasgow-haskell-users@haskell.org > | > https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fmail.haskell. > | org%2fcgi-bin%2fmailman%2flistinfo%2fglasgow-haskell- > | > users%0a&data=01%7c01%7csimonpj%40064d.mgd.microsoft.com%7c5d13dce17e0b47a802 > | > 6308d31bc02832%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=6snGzNQuPSFVsZ2SY > | fAxdSvo%2fjCOXvlq6cwzQM0L6iY%3d _______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users
