Avoid partial Data.List.last in autogenerated Paths_<pkg>.hs

[mpilgrem]

Motivation (in part):

• STAN complains about use of partial Data.List.last
• Add a suitable {-# WARNING in "x-partial" ... #-} to Data.List.{init,last} core-libraries-committee#292

I have reasoned that this minor refactoring is correct, but I am not sure how to test it. EDIT: What I have done is:

• unit tested lastChar in GHCi; and

• tested that a separate copy of the module does compile.

• Patches conform to the coding conventions.

• Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions). N/A

[Bodigrim]

Would it work if Cabal is driving a very old GHC (or non-GHC Haskell implementation) which does not yet have Data.List.NonEmpty in base? I think it's better to inline

lastM :: [a] -> Maybe a 
lastM = foldr (\x -> Just . fromMaybe x) Nothing

(The existence of fromMaybe is guaranteed by Haskell Report 2010)

[geekosaur]

I did think about that, but I also think nobody bothers to care about non-GHC any more (I think Data.List.NonEmpty is in all the ghcs we support). That ship sailed long ago, sadly.

[mpilgrem]

The stated GHC support window for Cabal is five years, which means GHC 8.8.2 (released 16 January 2020) on. GHC 8.8.2 comes with base-4.13.0.0, which exposes Data.List.NonEmpty. Data.List.NonEmpty joined base in 4.9.0.0 (GHC 8.0.1, released 21 May 2016).

The Cabal project does not identify any support for non-GHC compilers of Haskell.

[Bodigrim]

The stated GHC support window for Cabal is five years, which means GHC 8.8.2 (released 16 January 2020) on. GHC 8.8.2 comes with base-4.13.0.0, which exposes Data.List.NonEmpty.

My understanding always was that this is a support window for GHC to compile Cabal sources, not which GHC can be driven by Cabal. Changing Paths_<pkg>.hs affects the latter. Could this be clarified in CONTRIBUTING.md?

IIRC Cabal 3.12 was happily able to drive GHC 7.0 and such.

[geekosaur]

We stopped being able to test anything before 8.0 because ghcup's only 7-series for Ubuntu segfaults during install.

[mpilgrem]

@Bodigrim, you are correct - I misunderstood CONTRIBUTING.md (which is clear, now you point it out).

As we know that dir@(c:cs) is non-empty, we could use:

lastChar c [] = c
lastChar _ (c:cs) = lastChar c cs

[mpilgrem]

@geekosaur, second attempt at this submitted, hopefully meeting @Bodigrim's observation.

The 'required' CI 'Quick jobs / Meta checks' is failing at step 'Check that diff is clean'. I do not know what that is testing, or why it is failing.

[geekosaur]

I saw that, and I don't know either (I can't figure out how a commit pushed to GitHub could fail that check??).

[geekosaur]

And I just checked out your branch locally, and it's clean here. (Both the diff against master (and HEAD^) and the git diff-files -p check that's failing.)

@Mikolaj, @Kleidukos do either of you know what's going on here?

[ulysses4ever]

@mpilgrem you'll have to update https://github.com/haskell/cabal/blob/master/templates/Paths_pkg.template.hs according to your changes to Z.hs. See, for example, the previous instance of such a coordinated update: cc8d5df

[Mikolaj]

Would it work if Cabal is driving a very old GHC (or non-GHC Haskell implementation) which does not yet have Data.List.NonEmpty in base?

Thinking aloud, if it's driving a compiler, not compiling with it, NonEmpty should work fine regardless whether it's supported by the compiler. What would matter if some compiler options were missing, but that's very unrelated.

Also thinking aloud, tests would be great, but there's no way to test this, except for adding a STAN run to CI or checking for a new GHC warnings to the same effect (though I don't remember if the new warnings complain about last).

[mpilgrem]

@ulysses4ever, many thanks. This (final) version now passes the CI.