What Haskell users are actively maintaining or deving software using ghc <8

[Andreas Abel]

@Carter: A hackage crawler could probably give a good answer to your 
original question.

PatternSynonyms were introduced in 7.8 (but maybe didn't work 100% from 
the start).  I also consider PatternSynonyms a major improvement; 
although the "proper" way of library design would be not to export 
constructors in the first place, but use abstract data types.  In 
practice, few have the discipline, though.

GHC development is unfortunately not monotone, e.g. here is a bug 
introduced in 8.4 and fixed in 8.8:

   https://github.com/agda/agda/issues/4100

A solution here would be to backport bug fixes, but the community does 
not have the resources to do this.

 > To be fair, it’s way easier to do meticulous ci across a whole matrix 
of ghc versions now. But it is still a no zero cost on maintainers.

I agree.

In general, Haskell is a very nervous language, with lots of changes all 
the time.  This seems to be the fate of a language that is both a 
science lab for programming language research and a language people use 
for serious development.
I'd hope for a slower breakage rate on the side of the syntax and the 
standard library.

I also hope that Haskell 2020 will surface and cut a bit off the 
language extension forest.


On 2020-05-27 22:08, Carter Schonwald wrote:
> When it’s super easy to support a wide ghc range I totally support it!
> 
> That said , there are absolutely compiler bugs that are terrible in 
> older ghc.  Eg in 7.6, 7.8, and 7.10.1 and 7.10.2 (fixed in 7.10.3), 
> there’s a really nasty bug in the register allocator that mixed up float 
> and double.  Though I can’t find the ticket atm.
> 
> One point I realized / articulated recently is that supporting pre ghc 
> 8.0 makes it difficult to change public data  types without breaking all 
> current users.  Which is something pattern synonyms supports very well 
> (I dislike how it interacts with coverage checking, but that’s a whole 
> bother ball of wax. )
> 
> There’s often a very real cost to supporting ever widening ranges that 
> cover larger and large range of versions and dialects and bug 
> workarounds.  5 years ago supporting just the three most recent ghc 
> major versions was considered amazing. To be fair, it’s way easier to do 
> meticulous ci across a whole matrix of ghc versions now. But it is still 
> a no zero cost on maintainers.
> 
> 
> 
> On Wed, May 27, 2020 at 2:04 PM Vanessa McHale <vamchale at gmail.com 
> <mailto:vamchale at gmail.com>> wrote:
> 
>     I don’t drop support (that would be silly) as often as I write a new
>     library that needs GHC >= 8.0 or sometimes GHC>7.6
> 
>     I use newer versions of base for unsafeDupablePerformIO and some
>     lazy ST monad features I think
> 
>     I’d definitely consider older GHCs if there’s enthusiasm.
> 
>     Cheers,
> 
>      > On May 27, 2020, at 12:55 PM, Andreas Abel
>     <andreas.abel at ifi.lmu.de <mailto:andreas.abel at ifi.lmu.de>> wrote:
>      >
>      > I am using ghc 7.6 and up.  I dropped support for ghc 7.4 and
>     below because I fancy
>      >
>      >  \case
>      >
>      > which is new in 7.6.
>      >
>      > I'd say there is little reason to support ghc 6 any longer (but
>     probably this is anyway the consensus).
>      >
>      > That said, I do not maintain any libraries.  Libraries should not
>     drop ghc versions lightly.
>      >
>      > On 2020-05-27 16:50, Carter Schonwald wrote:
>      >> Hey all,
>      >> What are the oldest ghc versions folks are actually using to
>     build software they actually use ? What are the contexts for these ?
>      >> I know a lot of library maintainers, myself included try to make
>     it easy to suport as wide a version range of ghc as possible.  In my
>     case I find it useful to just have another way to evaluate how
>     stable I can make a library.
>      >> That said, what actual old ghc versions are folks actually using?
>      >> Afaict, the oldest ghc currently in a lts linux distro is ghc
>     7.0 in centos 6
>      >> Then centos 7 and the oldest Ubuntu lts are 7.6, then more
>     recent distros plus most other os platforms like the bsds are on
>     8.0-8.4 as the oldest supported / provided ghc.
>      >> Who are the users today and how important are they for todays
>     library maintainers ?
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