Re: Breaking changes to the base library

20 Jun 2021


      Just an aside. I looked up the GHC user guide and found this:

-Wcompat-unqualified-imports

Warns on qualified imports of core library modules which are subject to
change in future GHC releases.
I got confused by the text. I guess it was meant to be "Warns on
unqualified imports".
-harendra

On Sun, 20 Jun 2021 at 20:39, Oleg Grenrus  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  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  wrote:
...
Harendra Kumar  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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