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@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@smart-cactus.org> wrote:
Harendra Kumar <harendra.kumar@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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