[ghc-steering-committee] #682: Explicit Level Imports, recommendation: accept
Arnaud Spiwack
arnaud.spiwack at tweag.io
Wed Feb 5 06:20:32 UTC 2025
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
>>
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--
Arnaud Spiwack
Director, Research at https://moduscreate.com and https://tweag.io.
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