[ghc-steering-committee] #682: Explicit Level Imports, recommendation: accept
Simon Marlow
marlowsd at gmail.com
Wed Feb 5 15:37:11 UTC 2025
I don't feel strongly about 1 vs 2 extensions, because I think the
direction of travel is away from recommending individual extensions as a
way to interact with the compiler and towards GHC20xx instead. And in
GHC20xx I think we would enable either both or neither of these extensions,
correct?
Cheers
Simon
On Wed, 5 Feb 2025 at 08:16, Erik de Castro Lopo <erikd at mega-nerd.com>
wrote:
> Hi,
>
> I have read through the proposal, but there is something I am still unsure
> of. For the LANGUAGE pragma's is there any utility in using one separately
> form the other? It seems there isn't. In any one file you would use either
> one or the other.
>
> Thanks,
> Erik
>
> Arnaud Spiwack wrote:
>
> > Sorry I disappeared for a while. I second Simon's call, let's vote. Let
> me
> > repost a link to Simon's pro and cons post
> >
> https://github.com/ghc-proposals/ghc-proposals/pull/682#issuecomment-2609199731
> >
> > So far, we have the following votes
> >
> > - Simon: 1 extension
> > - Adam: 2 extension (feels quite strongly about it)
> > - Sebastian: 1 extension (on the Github thread, but I'll count it as a
> vote
> > anyway)
> >
> > Eric, Moritz, Malta, Matthías, Erik, Jakob: what do you think?
> >
> > ---
> >
> > Beyond that we have a single piece of community feedback on the Github
> > thread. It's from Michael Peyton Jones who is in favour of 2 extensions,
> > find it here
> >
> https://github.com/ghc-proposals/ghc-proposals/pull/682#issuecomment-2609583126
> >
> > ---
> >
> > For the record, I hadn't commented about it in my recommendation, despite
> > my well-known and desperately public distaste for micro extensions. I
> have
> > a couple of reasons:
> > - I dislike micro-extensions less now that we are doing the GHC20XX (the
> > last one was very modest, I'm in favour, by the way, of doing a much more
> > ambitious language edition soon, otherwise my distaste will come back
> with
> > a vengeance)
> > - While I consider every proposal with several extensions in it with
> > suspicion, the authors did argue for their second extension, I found the
> > argument mildly convincing, and thought it wasn't worth fighting against.
> >
> > Now, even like this my preference is mildly for one extension. Adam says
> > that it's easier to implement warnings with both the new syntax on and
> > implicit stage persistence left turned on, than to implement errors when
> > implicit stage persistence is turned off. It may be so, but I don't think
> > we can avoid implementing the errors anyway, so I don't feel that it's a
> > particularly compelling argument. I don't feel strongly. But that's
> > presumably where my vote goes.
> >
> > On Tue, 4 Feb 2025 at 07:13, Simon Peyton Jones <
> simon.peytonjones at gmail.com>
> > wrote:
> >
> > > Yes: all members of the steering committee, please vote. Evaluating
> > > proposals is what we all signed up to do!
> > >
> > > Thanks
> > >
> > > Simon
> > >
> > > On Mon, 3 Feb 2025 at 20:45, Adam Gundry <adam at well-typed.com> wrote:
> > >
> > >> I'm (unsurprisingly) in favour of acceptance, and I vote for two
> > >> extensions. As I commented on the GitHub thread:
> > >>
> > >> > We shouldn't unnecessarily conflate a syntactic extension
> > >> (ExplicitLevelImports) with a semantic one
> (NoImplicitStagePersistence)
> > >> just because the common case is to want both and we want to keep the
> > >> number of extensions lower.
> > >>
> > >> If there are reasons why having two extensions is actually
> problematic,
> > >> I'd like to hear them.
> > >>
> > >> Also, at the risk of opening another avenue of discussion, we also
> need
> > >> to resolve the syntax question (see
> > >>
> > >>
> https://github.com/ghc-proposals/ghc-proposals/pull/682#discussion_r1849123243
> ).
> > >>
> > >> I don't have a very strong opinion here, but given that some people
> do,
> > >> perhaps we should make ImportQualifiedPost affect splice imports so we
> > >> have
> > >>
> > >> import splice qualified A -- By default
> > >> import A splice qualified -- Under ImportQualifiedPost
> > >>
> > >> In any case, please do vote! It would be good to get this proposal
> done.
> > >>
> > >> Cheers,
> > >>
> > >> Adam
> > >>
> > >>
> > >>
> > >> On 27/01/2025 11:52, Simon Peyton Jones wrote:
> > >> > Arnaud
> > >> >
> > >> > OK, following my call and some further iteration, the proposal is
> much
> > >> > improved. See here
> > >> > <https://github.com/ghc-proposals/ghc-proposals/pull/682>. Please
> > >> read
> > >> > the new "Proposed Change Specification" which has had a large
> rewrite.
> > >> >
> > >> > I vote to accept.
> > >> >
> > >> > BUT there is one point that the committee must resolve: *one
> extension
> > >> > of two?* It's just a judgement call and I lay out the choices in
> this
> > >> > post
> > >> > <
> > >>
> https://github.com/ghc-proposals/ghc-proposals/pull/682#issuecomment-2609199731
> >.
> > >> I doubt that we'll get much community feedback. I suggest that we
> just
> > >> vote. I vote for one, not two. As does Sebastian.
> > >> >
> > >> > Over to you Arnaud. Let's get this one done. Matthew is busy
> > >> > implementing it for a customer and it has been on our to-do list for
> > >> > some time now. (Partly my fault.)
> > >> >
> > >> > Simon
> > >> >
> > >> > On Tue, 21 Jan 2025 at 10:48, Simon Peyton Jones
> > >> > <simon.peytonjones at gmail.com <mailto:simon.peytonjones at gmail.com>>
> > >> wrote:
> > >> >
> > >> > Matthew and I had a good conversation. Some notes here
> > >> > <
> > >>
> https://docs.google.com/document/d/1dEMPIHpbN19xYZymGCxO0BpQR58RPhk5MslruY7yXD0/edit?usp=sharing
> > >> >.
> > >> >
> > >> > He's going to work on a revision to the proposal which I'll
> iterate
> > >> > with him.
> > >> >
> > >> > Simon
> > >> >
> > >> > On Tue, 21 Jan 2025 at 07:37, Arnaud Spiwack
> > >> > <arnaud.spiwack at tweag.io <mailto:arnaud.spiwack at tweag.io>>
> wrote:
> > >> >
> > >> > Then, let's wait until your call with Matthew and decide
> how to
> > >> > act then.
> > >> >
> > >> > On Tue, 21 Jan 2025 at 02:43, Simon Peyton Jones
> > >> > <simon.peytonjones at gmail.com
> > >> > <mailto:simon.peytonjones at gmail.com>> wrote:
> > >> >
> > >> > Arnaud
> > >> >
> > >> > I have responded with a lot of feedback on the Github
> thread
> > >> > <
> > >>
> https://github.com/ghc-proposals/ghc-proposals/pull/682#pullrequestreview-2562175116
> > >> >.
> > >> >
> > >> > TL:DR: I like the direction of travel but have too many
> > >> > questions of detail to be ready to accept it just yet.
> > >> >
> > >> > I have arranged a call with Matthew.
> > >> >
> > >> > Simon
> > >> >
> > >> > On Thu, 9 Jan 2025 at 06:31, Arnaud Spiwack
> > >> > <arnaud.spiwack at tweag.io <mailto:
> arnaud.spiwack at tweag.io>>
> > >> > wrote:
> > >> >
> > >> >
> > >> > Mathew Pickering, Rodrigo Mesquita, and our own Adam
> > >> > Gundry put forward a new proposal for the perenial
> > >> > problem of dependencies and Template Haskell
> > >> >
> https://github.com/ghc-proposals/ghc-proposals/pull/682
> > >> > <
> > >> https://github.com/ghc-proposals/ghc-proposals/pull/682>
> > >> >
> > >> > I've got to be honest, I'm not fully convinced by
> the
> > >> > proposal. More on that in a minute, but it learns a
> > >> > lesson from previous attempts at the same problem by
> > >> > solving the absolute minimal problem, but this
> leads to
> > >> > a somewhat fork-like situation for which it isn't
> clear
> > >> > whether it will be resolved in the future. That
> being
> > >> > said, it solves a real problem which has plagued GHC
> > >> > compilation forever. And I'm inclined to believe
> that we
> > >> > can't really do much better.
> > >> >
> > >> > But I'm getting ahead of myself. The problem is that
> > >> > when you have -XTemplateHaskell in a file, all the
> > >> > dependencies' compiled code must suddenly be
> available
> > >> > for typechecking. This breaks `-fno-code` and wounds
> > >> > recompilation avoidance. This is probably the main
> > >> > reason why it's a widely held belief that Template
> > >> > Haskell is slow: you use Template Haskell in a few
> > >> > modules, and suddenly your IDE is much less
> responsive
> > >> > and you recompile more files. Yay?
> > >> >
> > >> > Anyway, the general gist of the solution is clear:
> we
> > >> > must be able to specify that we don't want to
> import a
> > >> > module for Template Haskell (there is subtleties in
> this
> > >> > too as you will want a little more control than
> that for
> > >> > cross-compilation reasons which I'm not competent
> about
> > >> > to comment on). But the devil is in the many
> details.
> > >> > There's this thing called implicit cross-stage
> > >> > persistence which says that anything you import
> > >> > not-for-template-haskell is going to be available in
> > >> > quotes and splices anyway. Sigh… So you have to turn
> > >> > this off. This is what the proposal does. And pretty
> > >> > much only.
> > >> >
> > >> > They introduce a new
> > >> > extension-XNoImplicitStagePersistence which disables
> > >> > that, and a little bit of syntax to specify the
> stage of
> > >> > imports. That's it.
> > >> >
> > >> > But it comes with severe limitations, most
> importantly:
> > >> > you can't ever use a symbol defined in the current
> > >> > module in a quote or splice of this current module,
> > >> > typed template Haskell is turned off.
> > >> >
> > >> > For these situations, the proposal kind of
> advertises
> > >> > using `-XImplicitStagePersistence`. Which does seem
> like
> > >> > a fork-like situation to me. Not cool. Yet… yet
> Template
> > >> > Haskell is a big messy ball of yarn, and I don't
> think
> > >> > it's fair to ask of any proposal to entangle it
> > >> > completely. The failure of past attempts seem to
> support
> > >> > this case. And I believe the authors are correct
> when
> > >> > they claim that this proposal, in practice, covers a
> > >> > vast majority of the uses of Template Haskell out
> there.
> > >> > So maybe we can see that as a new foundation for
> > >> > Template Haskell. I'm not thrilled about it, but
> it's
> > >> > probably the most reasonable way forward.
> > >> >
> > >> > The real problem with this sort of proposal is that
> then
> > >> > I get to write way too long an email to the
> committee.
> > >> > Hopefully this didn't deter you. Read the proposal,
> and
> > >> > let's vote.
> > >> >
> > >> > --
> > >> > Arnaud Spiwack
> > >> > Director, Research at https://moduscreate.com
> > >> > <https://moduscreate.com> and https://tweag.io
> > >> > <https://tweag.io>.
> > >>
> > >>
> > >> --
> > >> Adam Gundry, Haskell Consultant
> > >> Well-Typed LLP, https://www.well-typed.com/
> > >>
> > >> Registered in England & Wales, OC335890
> > >> 27 Old Gloucester Street, London WC1N 3AX, England
> > >>
> > >> _______________________________________________
> > >> ghc-steering-committee mailing list
> > >> ghc-steering-committee at haskell.org
> > >>
> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
> > >>
> > > _______________________________________________
> > > ghc-steering-committee mailing list
> > > ghc-steering-committee at haskell.org
> > >
> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
> > >
> >
> >
> > --
> > Arnaud Spiwack
> > Director, Research at https://moduscreate.com and https://tweag.io.
>
>
> --
> ----------------------------------------------------------------------
> Erik de Castro Lopo
> http://www.mega-nerd.com/
> _______________________________________________
> ghc-steering-committee mailing list
> ghc-steering-committee at haskell.org
> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-steering-committee/attachments/20250205/1474a38a/attachment-0001.html>
More information about the ghc-steering-committee
mailing list