Reconsidering -Wall and -Wcompat

Sven Panne svenpanne at
Sun Feb 14 18:51:19 UTC 2016

2016-02-14 17:12 GMT+01:00 Ben Gamari <ben at>:

> [...] 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.

IMHO, the distinction between "during development" and "outside of it" is
purely hypothetical.  A typical workflow is: Develop your code locally
against one GHC/set of libraries, commit to GitHub and let Travis CI do the
real work of testing against a matrix of configurations. If things work
well and the changes are worth it, tag your current state and release it.
Where exactly in this scenario is the code leaving the "during development"
state? I definitely want to enable -Wall for the Travis CI builds, because
that's the place where they are most valuable. As stated on the Wiki, stuff
in -Wcompat will often be non-actionable, so the only option I see if
-Wcompat is included in -Wall will be -Wno-compat for all my projects.
-Wcompat would be restricted to a few manual local builds to see where
things are heading.

> 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, [...]

Seeing -Wall this way is quite unusual, especially for people coming from
C/C++ (and the numbers quoted from Hackage seem to be a hint that others
think so, too). Normally, -Wall -Wextra -pedantic etc. are life-savers and
should be kept enabled all the time, unless you like endless debugging
hours, of course.

In a nutshell: I would consider -Wall implying -Wcompat an annoyance, but
as long as it can be disabled by 2 lines in .travis.yml, I don't really
care. ;-)

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