Breaking changes to the base library

[Chris Smith]

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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