Breaking changes to the base library

[Oleg Grenrus]

What is the reason that -Wcompat flags are not enabled by -Wall? The
unqualified Data.List import was warned by -Wcompat about since
GHC-8.10, I'm surprised that it still surprise people.

Should -Wcompat be implied by -Wall? Why not? I think that people who
won't ever consider upgrading GHC to be a very very small minority, and
they can suppress these warnings with `-Wall -Wno-compat`.

- Oleg

On 20.6.2021 17.57, Edward Kmett wrote:
> The breakage concern is why Data.List wound up in its limbo-like state
> of re-exporting the Foldable-polymorphic combinators since 7.10 --
> while "weird", it was the only option that didn't have to choose
> between removing the names from Data.List exports entirely and
> breaking unqualified imports of Data.List.
>
> With the monomorphized combinators in place, Data.List should be
> considered a 'qualified' import like Data.Map. We definitely need to
> do more to communicate that this is changing and how users should
> adjust their code to suit. After all, by far the most common intended
> import from Data.List is the humble 'sort', which doesn't conflict.
>
> -Edward
>
> On Sun, Jun 20, 2021 at 4:45 AM Chris Smith <cdsmith at gmail.com
> <mailto:cdsmith at gmail.com>> wrote:
>
>     Yikes, this is going to break nearly everything.  Definitely good
>     to let people know.
>
>     On Sun, Jun 20, 2021 at 7:43 AM Ben Gamari <ben at smart-cactus.org
>     <mailto:ben at smart-cactus.org>> wrote:
>
>         Harendra Kumar <harendra.kumar at gmail.com
>         <mailto:harendra.kumar at gmail.com>> writes:
>
>         > I see the following errors when compiling with ghc head version:
>         >
>         > $ ghc-stage2 --version
>         > The Glorious Glasgow Haskell Compilation System, version
>         9.3.20210608
>         >
>         > $ cabal build --with-compiler ghc-stage2 --allow-newer
>         >
>         > Data/Colour/CIE.hs:80:12: error:
>         >     Ambiguous occurrence ‘sum’
>         >     It could refer to
>         >        either ‘Prelude.sum’,
>         >               imported from ‘Prelude’ at
>         Data/Colour/CIE.hs:25:8-22
>         >               (and originally defined in ‘Data.Foldable’)
>         >            or ‘Data.List.sum’,
>         >               imported from ‘Data.List’ at
>         Data/Colour/CIE.hs:41:1-16
>         >               (and originally defined in ‘GHC.List’)
>         >    |
>         > 80 |    total = sum $ map fst l
>         >    |            ^^^
>         >
>         > Can someone briefly describe this change and what's the
>         recommended way of
>         > fixing this? Just hide the Data.List definition? I do not
>         see this
>         > mentioned in the release notes of 9.2/9.4 here:
>         >
>         https://ghc.gitlab.haskell.org/ghc/doc/users_guide/9.2.1-notes.html
>         >
>         Indeed, this is due to the monomorphic Data.List proposal,
>         which the
>         CLC decided would accompany the addition of
>         Data.List.singleton. The
>         correct fix here is to either qualify the import of
>         `Data.List` or add
>         an explicit import list. I'll try to remember to add a note
>         about this
>         to the release notes and migration guide.
>
>
>         Cheers,
>
>         - Ben
>
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