Great thanks Edsko -- v helpful.
By the way, I am not entirely sure why the proposal mentions the IsStatic class
> explicitly, but I just wanted to mention that it is really quite useful; I
> declare two instances in my Unfolder episode :)
The proposal mentions IsStatic only because I wanted to give the expansion
that GHC performs. I agree that it's useful, but it's really orthogonal
to the proposal.
I'll look out for the Unfoldr episode -- if you remember do send me a link
Thanks again
Simon
On Fri, 9 Jan 2026 at 12:25, Edsko de Vries <edsko(a)well-typed.com> wrote:
> Hi Simon,
>
> Apologies for my slow reply. It's been nearly 15 years (!) since I wrote
> distributed-process and distributed-static (fun fact: it was my first
> task when I joined Well-Typed) and I felt I needed to set aside some time
> to properly page everything back in, as I haven't really used static
> pointers ever since (somewhat sadly..). I realize that the proposal how now
> been written and accepted, so this email comes too late in a way, but I
> finally had some time now to look at this properly. In hindsight, the
> proposal is simple enough, but I didn't realize that at the time.
>
> Anyway. The proposal is fine with me :) I seems to me that it's no more
> than a minor inconvenience to the programmer. I have now prepared a Haskell
> Unfolder episode (which will likely be the next episode we air, episode
> #53) on static pointers, where I discuss the primitive building blocks
> provided by ghc, and show how we can implement some compositional
> infrastructure around that (including some thoughts on type classes, which
> I believe you and I discussed a very long time ago). I'll also mention the
> proposal and the upcoming changes in 9.14.2 (warning) and 9.16 (change
> implemented). I tried building it with your branch (wip/T26718) and that
> all worked fine. I also added one module in which your branch issues the
> warning expect, and that seemed good also.
>
> By the way, I am not entirely sure why the proposal mentions the IsStatic
> class explicitly, but I just wanted to mention that it is really quite
> useful; I declare two instances in my Unfolder episode :)
>
> -Edsko
> On 11/19/25 13:02, Simon Peyton Jones wrote:
>
> Thanks Laurent and Mathieu
>
> *Edsko, Duncan, Facundo*: do you have any views?
>
> I have now written a GHC proposal
> <https://github.com/ghc-proposals/ghc-proposals/blob/wip/spj-static/proposal…>.
> Could you add your thoughts to it? I think it's a no-brainer myself, but
> we should go through the proper process.
>
> Do any of you know people I should consult? Eg authors of libraries that
> use StaticPtrs?
>
> Thanks
>
> Simon
>
>
>
> On Fri, 7 Nov 2025 at 21:18, Simon Peyton Jones <
> simon.peytonjones(a)gmail.com> wrote:
>
>> Dear Laurent, Duncan, Mathieu, Facundo, Edsko
>>
>> I have spent a little while digging into *static pointers* recently.
>> See my post below. I wonder if you have any comments on my proposal?
>>
>> Do you know anyone else I should consult?
>>
>> Thanks!
>>
>> Simon
>>
>> On Fri, 7 Nov 2025 at 18:13, Simon Peyton Jones (@simonpj) <
>> gitlab(a)gitlab.haskell.org> wrote:
>>
>>> Simon Peyton Jones <https://gitlab.haskell.org/simonpj> created an
>>> issue: #26556 <https://gitlab.haskell.org/ghc/ghc/-/issues/26556>
>>>
>>> Static pointers are not properly implemented. For example:
>>>
>>> - #26545 <https://gitlab.haskell.org/ghc/ghc/-/issues/26545>
>>> - #24464 <https://gitlab.haskell.org/ghc/ghc/-/issues/24464>
>>> - #24773 <https://gitlab.haskell.org/ghc/ghc/-/issues/24773>
>>>
>>> among others. Moreover, the implementation is very messy, scattered
>>> about in FloatOut and elsewhere.
>>>
>>> Let's fix it.
>>> Discussion
>>>
>>> I embarked on what I thought would be a simple refactor to
>>>
>>> - Identify static bindings in the type checker
>>> - Promote them to top level desugarer
>>>
>>> thereby avoiding all the terribly painful static-form-floating stuff
>>> that been an ongoing source of breakage and irritation.
>>>
>>> Sadly it was not as simple as I had thought. Merge request !14994
>>> <https://gitlab.haskell.org/ghc/ghc/-/merge_requests/14994> is my work
>>> in progress
>>>
>>> -
>>>
>>> At first it seems simple: given static e
>>> - When typechecking e ensure that all its free variables are
>>> top-level defined
>>> - When desugaring, move e to top level
>>>
>>> Apparently simple!
>>> -
>>>
>>> *Complication 1*. e might generate constraints. We don't want to
>>> solve those from locally-bound Givens, because they'll be out of scope when
>>> we promote to top level.
>>>
>>> Solution: wrap the constraints in an implication with SkolInfo of
>>> StaticFormSkol; and in the constraint solver zap all Givens when
>>> walking inside such an implication. That was done in
>>>
>>> commit 39d4a24beaa7874a69ffdc1528ca160818829169Author: Simon Peyton Jones <simon.peytonjones(a)gmail.com>Date: Tue Sep 30 23:11:19 2025 +0100 Build implication for constraints from (static e) This commit addresses #26466, by buiding an implication for the constraints arising from a (static e) form. The implication has a special ic_info field of StaticFormSkol, which tells the constraint solver to use an empty set of Givens.
>>>
>>> So that complication wasn't at all bad.
>>> -
>>>
>>> *Complication 2*. What if we have
>>>
>>> f x = let y = reverse "hello" in ...(static (y++y))...
>>>
>>> The free vars of the static are just {y}, and y is
>>> morally-top-level. It in turn has no free variables.
>>>
>>> Sadly (as it turns out) GHC tries to accept this case. When looking
>>> at the defn of y (with no static in sight yet) the typechecker marks
>>> it at a "static binding", meaning that it too can (and indeed must) be
>>> floated to top level.
>>>
>>> So if the desugarer moves the static to the top level, it must move y
>>> too. And that means it must mark the typechecked binding in some way, so
>>> the desugarer can identify it. Not so hard, but there is quite a bit of new
>>> plumbing.
>>> -
>>>
>>> *Complication 3*. But what if y's RHS generates constraints, which
>>> use Givens (or solved dictionaries, which are very similar) from its
>>> context. E.g.
>>>
>>> f x = let p = x+1::Int; y = 2+3::Int in ...
>>>
>>> Now there may be a d :: Num Int lying around from dealing with p,
>>> and y may use it. Oh no! Now that'll be out of scope if we move y to
>>> top level.
>>>
>>> Plausible solution: use them same mechanism for static bindings as
>>> we did for static e expressions. That is, build an implication
>>> constraint whose SkolInfo says "zap Givens". This turned out to be
>>> considerably harder to implement than it was for Complication 1.
>>> -
>>>
>>> *Complication 4*. What if y is not generalised, perhaps because of
>>> the Monomorphism Restriction? e.g.
>>>
>>> f :: Num a => a -> blahf x = let y = 3+3 in (x+y, static( ..y.. ))
>>>
>>> Now y is monomorphic and really does use the dictionary passed to f.
>>> So it really cannot appear in the static. Somehow y really isn't
>>> static after all. We must reject this program. Not only is it an
>>> implementation mess (Complications 1,2,3 are already imposing quite a
>>> signficant implemenation burden) but it becomes pretty hard to explain to
>>> the programmer just which uses of static are OK and which are not.
>>>
>>> What a swamp. At this point I threw up my hands and wrote this
>>> summary
>>>
>>> Proposal
>>>
>>> To me the solution is clear: the rule should be
>>>
>>> - *in static e, all the free vars of e should be bound at top level*
>>>
>>> That is a nice simple rule; it is easy to explain and easy to implement.
>>> It is also what the user manual says!
>>>
>>> In retrospect, by addressing Complication 2 I was trying too hard! (And
>>> this extra feature is entirely undocumented.) I thought that I could deal
>>> with Complication 2 using the same mechanism as the one that deals with
>>> MonoLocalBinds. But I was wrong.
>>>
>>> Making this change could perhaps break some programs. They would all be
>>> easy to fix, by moving bindings for any free variables to top level. But
>>> note that the status quo is not stable: it has bugs e.g #24464
>>> <https://gitlab.haskell.org/ghc/ghc/-/issues/24464>, #26545
>>> <https://gitlab.haskell.org/ghc/ghc/-/issues/26545>. What we have is at
>>> attempt to be clever that is simply wrong.
>>>
>>> —
>>> View it on GitLab <https://gitlab.haskell.org/ghc/ghc/-/issues/26556>.
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>>
Hello all,
Unfortunately we discovered a major code generation regression
in GHC 9.12.3 that results in incorrect runtime results for
certain operations involving sub-word divisions.
We recommend avoiding upgrading to this minor release at this
time.
See https://gitlab.haskell.org/ghc/ghc/-/issues/26711 for more
details.
This is the same bug that affected GHC 9.12.1. A patch re-introducing
this bug was accidently backported onto the 9.12.3 branch
and went unnoticed throughout the testing period.
We are taking steps to ensure similar situations are less likely
to occur again, including looking at ways to improve our testing procedure
for minor GHC release, potentially using the Stackage infrastructure
to run the testsuites of a broad range of packages to detect
such issues.
We are also starting to prepare GHC 9.12.4 with a fix, which
we hope to release soon. However, a significant amount of time
has passed since the initial set of backports to GHC 9.12.3
were completed, which means that a number of merge requests
addressing issues of various severities has been marked for
backporting to the GHC 9.12 branch. We are in the process of
triaging this list and preparing a robust and reliable GHC 9.12.4
release.
Thanks,
Zubin.
Zubin
The accepted GHC proposal 732: Simplify static forms
<https://github.com/ghc-proposals/ghc-proposals/pull/732> needs a
deprecation warning added to GHC 9.14, to warn about static forms that
mention nested free variables.
Ticket #26718 <https://gitlab.haskell.org/ghc/ghc/-/issues/26718> tracks
the warning.
I have implemented that warning in branch
wip/T26178
in the main GHC repo. It's based on the ghc-9.14 branch.
I'm not sure how to run CI on it.
Can I hand it off to you now? It should be part of the 9.14.2 release as
agreed by the GHC steering committee.
RSVP
Thanks
Simon
