Reconsidering -Wall and -Wcompat

Manuel M T Chakravarty chak at
Sun Feb 14 23:04:45 UTC 2016

Just as one data point, the Swift compiler is by default showing warnings about upcoming changes. Just like deprecation warnings, I do find that helpful. Based on that experience, including -Wcompat in -Wall seems like a good plan to me.


> Ben Gamari <ben at>:
> tl;dr. GHC has a new set of warnings, -Wcompat, intended to give users
>       advance notice of coming library changes. We want to know whether
>       you think this set should be included in -Wall. See the Wiki [4]
>       and voice your opinion via the linked poll.
> Hello everyone,
> GHC 8.0.1 will include a new warning group, -Wcompat, which arose out of
> the MonadFail proposal discussion [1] late last year. This warning set
> is intended to provide a means of informing users of coming changes in
> GHC's core libraries.
> We would like to solicit the community's feedback on whether this new
> flag set should be implied by -Wall.
> This proposal is motivated by concern expressed by some that -Wcompat
> would see little usage unless it is placed in one of the warning sets
> typically used during development. One such set is -Wall, which enables
> a generous fraction of GHC's warning collectionand is is intended [2]
> for use during development.
> Unfortunately, despite the (albeit only recently stated) intent of
> flag, -Wall is widely used outside of development [3], often with the
> expectation that the result be warning-clean across multiple GHC
> versions. While we hope that -Wall will see less use in this context in
> the future, given its current role we wouldn't want to add options to it
> that would cause undue burden for users.
> So, we are asking for your opinion: should -Wcompat be implied by -Wall?
> You can find a more thorough description of the precise proposal on the
> GHC Wiki [4]. It would be very much appreciated if you could take a few
> minutes familiarize yourself with the options and provide your thoughts
> via this quick poll,
> Feel free to discuss the issue further in this thread.
> Thank you for sharing,
> - Ben
> [1]
> [2]
> [3] In a rather unscientific study, nearly half of packages on Hackage
>    include it in ghc-options,
>        $ tar -xf ~/.cabal/packages/00-INDEX.tar
>        $ (for pkg in $(ls); do ls $pkg/*/*.cabal | sort -r | head -n1; done) | xargs grep -L '\-Wall' | wc -l
>        4352
>        $ ls | wc -l 
>        9347
> [4]
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