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

Tue Jan 14 07:42:18 UTC 2025

Sebastian writes:

> I don't agree that -XNoImplicitStagePersistence is a fork. After all,
users are not forced to use `-XNoImplicitStagePersistence` just because one
of its imports uses it.

This isn't what we mean by forklike in our guidelines. We mean to avoid
situations where the same code means different things depending on the
extensions turned on and/or needing different modules having incompatible
sets of extensions. Our README reads:

By a "fork" we mean

   - It fails the test "Is this extension something that most people would
   be happy to enable, even if they don't want to use it?";
   - And it also fails the test "Do we think there's a reasonable chance
   this extension will make it into a future language standard?"; that is, the
   proposal reflects the stylistic preferences of a subset of the Haskell
   community, rather than a consensus about the direction that (in the
   committee's judgement) we want to push the whole language.

The idea is that unless we can see a path to a point where everyone has the
extension turned on, we're left with different groups of people using
incompatible dialects of the language. A similar problem arises with
extensions that are mutually incompatible.

I don't think this passes the first test, but it does pass the second
(though that future is probably quite far!). And I think that the
proposition that there's no way to make what we want of Template Haskell
without breaking the first test is reasonable (see also Adam's email
above). But still, this does create a forky situation for us, which I
wouldn't be doing my job as a shepherd if I wasn't pointing it out.
/Arnaud

On Mon, 13 Jan 2025 at 22:31, Matthías Páll Gissurarson <mpg at mpg.is> wrote:

> I vote accept.
>
> The proposal itself is well written, and clarifies the concepts involved
> and the issue at hand.
>
> I am on the fence with the syntax itself. I like the one presented in the
> proposal, it's very clean. I was a bit worried at first with having to
> import the same module multiple times at different levels, but I guess that
> cannot really be avoided.
> I like Richard's comment on having different sections, a `splice` section,
> a level 0 section and `quote` section. I'm also not against the `{-# SPLICE
> #-}` syntax if we decide to go down that route, but it's a bit grittier
> than the keywords.
>
>
>
> On Mon, 13 Jan 2025 at 09:27, Sebastian Graf <sgraf1337 at gmail.com> wrote:
>
>> Hi,
>>
>> I vote to accept this proposal.
>>
>> I would have liked to see a clear specification of what gets compiled
>> when with -XImplicitStagePersistence, but I see that this isn't strictly
>> necessary to describe the extension in terms of the Haskell-the-language,
>> plus it's quite complicated. I attempted to do so at the end of this post
>> <https://github.com/ghc-proposals/ghc-proposals/pull/682#pullrequestreview-2448500943>
>> .
>>
>> I don't agree that -XNoImplicitStagePersistence is a fork. After all,
>> users are not forced to use `-XNoImplicitStagePersistence` just because one
>> of its imports uses it.
>>
>> Cheers,
>> Sebastian
>>
>> Am Fr., 10. Jan. 2025 um 12:20 Uhr schrieb Moritz Angermann <
>> moritz.angermann at gmail.com>:
>>
>>> Hi all,
>>>
>>> I'm generally in support of this proposal. As many of you know, I
>>> strongly believe TemplateHaskell is a major wart that Haskell has, for many
>>> reasons. This proposal tries to address at least one of those: adding
>>> more clarity and explicitness about dependencies. It may help with
>>> cross compilation in that we have a clearer idea of what we exactly need
>>> to load in iserv (alternatives where we implicit link a runnable for
>>> target evaluation, can rely on dead code elimination for this, but
>>> having this from the start would already be helpful).
>>>
>>> I've recently been looking a lot at Zig's comptime, as they seem to have
>>> gone down almost the same route. Maybe there's some inspiration to
>>> be drawn from Zig's solution in the future. It is, however, WAY more
>>> restrictive than what we currently have in the form of TemplateHaskell.
>>>
>>> +1 on this one.
>>>
>>> Best,
>>>  Moritz
>>>
>>> On Fri, 10 Jan 2025 at 18:19, Adam Gundry <adam at well-typed.com> wrote:
>>>
>>>> Thanks Arnaud! With my "proposal co-author" hat on, I'd like to make a
>>>> few points inline...
>>>>
>>>>
>>>> On 09/01/2025 06:34, Arnaud Spiwack wrote:
>>>>  >
>>>> > On Thu, 9 Jan 2025 at 15:31, Arnaud Spiwack <arnaud.spiwack at tweag.io
>>>> > <mailto:arnaud.spiwack at tweag.io>> wrote:
>>>> >
>>>>  >     [...]
>>>> >
>>>> >     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.
>>>>
>>>> Regarding typed TH, the proposal currently grants a bit of flexibility
>>>> to the implementation in suggesting that TTH might not be supported at
>>>> all, primarily because TTH has some existing unresolved issues around
>>>> constraints. We could alternately say that TTH remains available (but
>>>> also remains somewhat broken, because fixing it is out of scope of the
>>>> implementation of this proposal).
>>>>
>>>>
>>>> >     For these situations, the proposal kind of advertises using
>>>> >     `-XImplicitStagePersistence`. Which does seem like a fork-like
>>>> >     situation to me. Not cool.
>>>>
>>>> Rather than seeing ImplicitStagePersistence as creating a language
>>>> fork,
>>>> I see it as necessary for backwards compatibility, but with the
>>>> intention that in the long term NoImplicitStagePersistence is the way
>>>> to
>>>> go. This may still be difficult in some cases (e.g. codebases that make
>>>> heavy use of Lift), but the idea is to start with a simple, restrictive
>>>> baseline (NoImplicitStagePersistence) and then gradually add features
>>>> relaxing this as needed (ExplicitLevelImports being the first of these,
>>>> but perhaps later something for multiple levels within a single file).
>>>>
>>>> Cheers,
>>>>
>>>> Adam
>>>>
>>>>
>>>> --
>>>> 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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>
>
> --
> --  Matthías Páll Gissurarson <http://mpg.is/>
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-- 
Arnaud Spiwack
Director, Research at https://moduscreate.com and https://tweag.io.
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