Scoped Type Variables discussion forum [was: open up the issues tracker on ghc-proposals]

Carter Schonwald carter.schonwald at
Mon May 21 00:21:49 UTC 2018

No. I’m saying make same variables get the parent quantified, even if it’s

Breaking changes are ok if they make things better.

Measuring impact really comes down to making the patch and measuring. It
will be an easy to fix breaking change and my experience has been that
teams in an industrial context are a ok with none silent breaking changes.
This would be very easy to catch :)

On the pedagogy side it actually makes learning simpler afaict :)

On Sun, May 20, 2018 at 8:03 PM Anthony Clayden <
anthony_clayden at> wrote:

> On Mon, 21 May 2018 at 11:23 AM, Carter Schonwald <redirect at>
> wrote:
>> indeed .. and we can reasonably say "lets deal with the bandaid in one go
>> by cleaning it up  in the next standard"
> Thanks Carter/Brandon, the reason for asking how we should go about the
> discussion was exactly: where/how are we going to maximise the chance of
> finding out what code is out there, and how disruptive any 'clean up' might
> be?
> Ghc has occasionally made breaking releases (not saying that's what we
> want to do), usually with some advance warning/deprecation period. And
> generally the Haskell community has accommodated that with understanding of
> the decision, to fix their code.
>> so what would the next gen look like?
>> eg: fresh variables get the usual implicit forall at the front of the
>> type, and everything else either needs an explicit quantifier OR it refers
>> to the outer implicit quantified variable?
> I wanted to avoid discussing 'next gen' in possibly-obscure/write-only
> mailing lists; and preferably get the discussion where posterity can see
> the reasoning/examination of options.
> "fresh variables get the usual implicit forall" is what happens now. (It's
> just that the User Guide explains it really badly -- I'm trying to fix that
> separately .) If the outer
> variable(s) are not explicitly forall-quantified, then even same-named
> variables count as fresh. IOW merely putting a `forall` can change the
> meaning of a program -- that's what causes the most confusion (judging by
> SO).
> The exception is variables bound in a pattern signature for an
> existentially-quantified data constructor: they *must* be fresh. This is
> hard for a reader to follow because the pattern signature with data
> constructor looks the same whether or not the constructor is
> existentially-quantified. What's worse a constructor might have a mix of
> existential and universal variables.
> AntC
>> On Sat, May 19, 2018 at 2:56 PM, Brandon Allbery <allbery.b at>
>> wrote:
>>> On Sat, May 19, 2018 at 7:32 AM, Anthony Clayden <
>>> anthony_clayden at> wrote:
>>>> So the explanation I've seen for the current design is it was deliberately idiosyncratic, to minimise any disruption to existing code. Then I'm asking whether any of that code is still around? If not/if it's been re-factored to use ScopedTypeVariables, then any tweak to the design could have a freer hand.
>>> The reason there's no discussion about that is that nobody here has the
>>> ability to go hunt down every last piece of code in every public or private
>>> (think Standard Chartered, Facebook, etc.) code base and its current owner,
>>> and order them to "fix" it. You can't win that battle.
>>> --
>>> brandon s allbery kf8nh                               sine nomine
>>> associates
>>> allbery.b at
>>> ballbery at
>>> unix, openafs, kerberos, infrastructure, xmonad
>>> _______________________________________________
>>> Glasgow-haskell-users mailing list
>>> Glasgow-haskell-users at
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