Re: [ghc-steering-committee] #682: Explicit Level Imports, recommendation: accept

OK, slightly in favor of two extensions. Erik Erik de Castro Lopo 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:

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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-2609199...

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?

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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-2609583...

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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 wrote:

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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 wrote:

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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_r18491232...).

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 mailto:simon.peytonjones@gmail.com> wrote:

Matthew and I had a good conversation. Some notes here < https://docs.google.com/document/d/1dEMPIHpbN19xYZymGCxO0BpQR58RPhk5MslruY7y... .

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 mailto:arnaud.spiwack@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 mailto:simon.peytonjones@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-25... .

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 mailto:arnaud.spiwack@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/

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-- Arnaud Spiwack Director, Research at https://moduscreate.com and https://tweag.io.

-- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

-- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/