Breaking changes to the base library

[Carter Schonwald]

This is a good question / suggestion!

On Sun, Jun 20, 2021 at 11:09 AM Oleg Grenrus <oleg.grenrus at iki.fi> wrote:

> 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> 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> wrote:
>>
>>> Harendra Kumar <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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