Warnings, -Wall, and versioning policy

Simon Peyton Jones simonpj at microsoft.com
Wed Jan 13 12:43:58 UTC 2016

Thanks Gershom.  That all sounds fine to me.  

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.

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.


| -----Original Message-----
| From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of Gershom B
| Sent: 13 January 2016 02:20
| To: GHC users <glasgow-haskell-users at haskell.org>; ghc-devs at haskell.org;
| Edward Kmett <ekmett at gmail.com>; Herbert Valerio Riedel <hvr at gnu.org>; Simon
| Marlow <marlowsd at 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 (marlowsd at 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 at haskell.org
| >
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