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

Anthony Clayden anthony_clayden at
Mon May 21 00:03:12 UTC 2018

On Mon, 21 May 2018 at 11:23 AM, Carter Schonwald <redirect at>

> 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

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

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.


> 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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