[ghc-steering-committee] #283: Local modules (again), recommendation: accept

Spiwack, Arnaud arnaud.spiwack at tweag.io
Thu Oct 7 07:52:31 UTC 2021 

Dear all,

First sorry I'm a bit late on this, life got in the way.

I've got opinions from both Simons, Tom, and Alejandro. Eric, Joachim,
Vitaly, Vlad what do you think about all this?

---

If I were to summarise the discussion as I understand it, the major point
of discussion is the difference between local modules (which are pure
name-spacing entities) and global/top-level modules (which are compilation
units, hence can appear in import statements for dependency management).
Simon PJ has argued (in an off-band conversation with Richard and I) to
embrace the difference more, though in many places in the proposal, these
two kind of modules are used the same way, hence deserving the common name
of “module”. Simon Marlow is worried that the lack of distinction between
global module addressing and local module addressing in `A.B.C.x` can be
confusing (is `A.B` a global module, or is `A` the global module and `B.C`
the local module?).

(I'd personally argue, regarding this latter point, that it doesn't matter.
By the time we are able to name `A.B.C.x` in a file, global modules may
have been renamed, so we have changed a name representing a compilation
unit into a namespace, which may or may not be identical. The proposal
don't treat the two situations differently at all)

Another point of discussion which hasn't appeared on this list but has been
raised in the Github thread, is that the current proposal goes to great
length to make exporting `module M` more or less compatible with the
current behaviour giving it a very different semantic to exporting `module
qualified M` (these are currently items 4.iv, 4.v, and 4.vi in the
specification section of the proposal). Note that is not fully backwards
compatible and even with existing module can export more stuff. The
question is whether it is worth trying to match the existing behaviour of
`module M`, or whether a simpler notion which is in line with the behaviour
of `module qualified M` would be preferable.

Over to you :-)

/Arnaud

On Tue, Jul 27, 2021 at 6:01 PM Simon Marlow <marlowsd at gmail.com> wrote:

> On Mon, 26 Jul 2021 at 18:08, Richard Eisenberg <rae at richarde.dev> wrote:
>
>>
>> One idea could be a new import item `import module M impspec` which
>> behaves just like `module M impspec`, but without the use of `qualify` in
>> its interpretation. In code:
>>
>> interpretImpItem('import' 'module' modids impspec, export_env)
>>   = interpretImpSpec(impspec, strip(modids, export_env))
>> interpretImpItem('import' 'module' modids, export_env)
>>   = strip(modids, export_env)
>>
>>
>> Now, we could say
>>
>> > import qualified Data.Set ( import module Set ) as S
>>
>> which would bring e.g. `S.Set` and `S.fromList` into scope.
>>
>> This new `import module` import item would enable this idiom. (This is
>> all possible with the proposal as stated, but not nearly as easily.) Would
>> that help? Then, someone who cared about the property you want would be
>> able to suggest a coding style that would maintain it.
>>
>
> As you say, you can do this using the proposal as is:
>
> module qualified S (module Set) where import Data.Set
>
> so I don't think it's worth adding anything new, given that this is
> already quite brief. (if perhaps a little obscure, but you could imagine
> getting used to it)
>
> But while thinking about this I realised I'm more concerned that we would
> have two kinds of qualification that behave in quite different ways, yet
> look identical. One is top-level modules, which you can bring into scoe and
> rename with import declarations, and the other is qualified exports which
> are controlled with export and import specifiers. When I refer to an
> identifier like A.B.C.x, zero or more of the modids come from the import
> declaration.
>
> This is powerful, yes, but also potentially a source of great confusion. I
> find it hard to predict exactly how this will work out in practice.
>
>
> Cheers
> Simon
>
>
>
>>
>> Richard
>>
>>
>>
>> Cheers
>> Simon
>>
>>
>> On Fri, 23 Jul 2021 at 11:00, Simon Marlow <marlowsd at gmail.com> wrote:
>>
>>> Just so I'm not completely silent: in the past I was generally in favour
>>> but had some suggestions. It looks like the proposal has undergone a lot of
>>> rewrites since I last reviewed it (or perhaps I just don't remember it all
>>> that well), I've started to go through it again but this is a biggie!
>>>
>>> I think a deadline is a good idea.
>>>
>>> Cheers
>>> Simon
>>>
>>> On Fri, 23 Jul 2021 at 07:23, Spiwack, Arnaud <arnaud.spiwack at tweag.io>
>>> wrote:
>>>
>>>> Dear all,
>>>>
>>>> I know that this proposal is a bit long, but it also deserves your
>>>> attention.
>>>>
>>>> I feel it's going to be easier to set a bit of time to review the
>>>> proposal if I give a deadline. So let's say the following: I'll be on
>>>> holiday starting two weeks from now (6th August), can I have everybody's
>>>> opinion by then?
>>>>
>>>> ---
>>>>
>>>> Recapitulating the opinions so far
>>>>
>>>>    - I'm personally pretty enthusiastic about the entire proposal
>>>>    - Tom voiced quite enthusiastic support for what Simon PJ calls
>>>>    (1), and (3)
>>>>    - Simon PJ wants (1), is not against (2), is mildly against (3)
>>>>    - Joachim suspends his judgement (which is fine, but hopefully not
>>>>    too many of us do this :-) ).
>>>>
>>>>
>>>> On Wed, Jul 21, 2021 at 2:30 PM Simon Peyton Jones <
>>>> simonpj at microsoft.com> wrote:
>>>>
>>>>> To be clear, I’m ok with (1), luke-warm on (2), and mildly against (3)
>>>>>
>>>>>    1. Import and export of qualified names. This seems like the Main
>>>>>    Point.
>>>>>    2. Local import (in a let/where). This seems low pain but low gain.
>>>>>    3. Local modules. This is the one I'm struggling with.
>>>>>
>>>>> There is  more on the (tail end of the) PR
>>>>> https://github.com/ghc-proposals/ghc-proposals/pull/283
>>>>>
>>>>>
>>>>>
>>>>> I am open to being educated.
>>>>>
>>>>>
>>>>> I would love to hear from other members of the committee.  Tom’s
>>>>> thumbs-up seemed to about (1), without saying anything about (2) and (3).
>>>>>
>>>>>
>>>>>
>>>>> One mechanism (if my categorisation is correct) could be to ask
>>>>> everyone to vote (yes/no/maybe) on all of 1,2,3.
>>>>>
>>>>>
>>>>>
>>>>> Arnaud, you are our shepherd.  Your sheep await your command.
>>>>>
>>>>>
>>>>>
>>>>> Simon
>>>>>
>>>>>
>>>>>
>>>>> *From:* ghc-steering-committee <
>>>>> ghc-steering-committee-bounces at haskell.org> *On Behalf Of *Richard
>>>>> Eisenberg
>>>>> *Sent:* 19 July 2021 21:18
>>>>> *To:* Spiwack, Arnaud <arnaud.spiwack at tweag.io>
>>>>> *Cc:* GHC Steering Committee <ghc-steering-committee at haskell.org>
>>>>> *Subject:* Re: [ghc-steering-committee] #283: Local modules (again),
>>>>> recommendation: accept
>>>>>
>>>>>
>>>>>
>>>>> Any thoughts on this? Simon PJ seems lukewarm (or maybe even cooler
>>>>> than that), Arnaud is in support, but the rest of you have been quiet.
>>>>>
>>>>>
>>>>>
>>>>> Thanks!
>>>>>
>>>>> Richard
>>>>>
>>>>>
>>>>>
>>>>> On Jun 11, 2021, at 3:05 AM, Spiwack, Arnaud <arnaud.spiwack at tweag.io>
>>>>> wrote:
>>>>>
>>>>>
>>>>>
>>>>> Dear all,
>>>>>
>>>>>
>>>>>
>>>>> Let me raise this proposal again. Very few of us have opined, and
>>>>> while I'd usually be happy to consider silence as assent, this is a rather
>>>>> large proposal which may require a few more pairs of eyes. Please consider
>>>>> giving this one a read and share your thoughts. If you can't do so right
>>>>> now, please let me know when you will be able to, so that we can plan
>>>>> accordingly.
>>>>>
>>>>>
>>>>>
>>>>> This is an important proposal, I'm keen on seeing its design finalised.
>>>>>
>>>>>
>>>>>
>>>>> /Arnaud
>>>>>
>>>>>
>>>>>
>>>>> On Wed, May 26, 2021 at 2:35 PM Richard Eisenberg <rae at richarde.dev>
>>>>> wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On May 26, 2021, at 3:28 AM, Spiwack, Arnaud <arnaud.spiwack at tweag.io>
>>>>> wrote:
>>>>>
>>>>>
>>>>>
>>>>> I'm realising that I inverted additional options 1 and 3 in my reply.
>>>>> To spell things out: I'm in favour of the namespace introduced for every
>>>>> datatype and such; and weakly in favour of anonymous modules, for which I
>>>>> prefer the `_` syntax than simply omitting the name.
>>>>>
>>>>>
>>>>>
>>>>> Oh, good. I was very confused here, but I decided not to push on it.
>>>>> I'm similarly weakly in favor of (1), but I can't get myself to decide
>>>>> firmly on whether to go with alternative (7). Going with (7) is a little
>>>>> more consistent with other features, but it adds more symbols to the source
>>>>> text that could otherwise be omitted. So I'm pretty ambivalent.
>>>>>
>>>>>
>>>>>
>>>>> Richard
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Tue, May 25, 2021 at 11:54 PM Richard Eisenberg <rae at richarde.dev>
>>>>> wrote:
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On May 25, 2021, at 3:09 PM, Alejandro Serrano Mena <trupill at gmail.com>
>>>>> wrote:
>>>>>
>>>>>
>>>>>
>>>>> - I am not sure of the benefit of allowing (1), compared with the
>>>>> possible surprise of users.
>>>>>
>>>>> - I do not fully understand (2).
>>>>>
>>>>> - I think (3) would be great, if we ensure that nothing changes if I
>>>>> don’t use “qualified”, even if -XLocalModules is on.
>>>>>
>>>>>
>>>>>
>>>>> If in the language, I would use (1) -- anonymous local modules --
>>>>> regularly, when defining a function or class instance with a bunch of
>>>>> "local" helper functions. Of course, if we can't omit the module name, I
>>>>> will suffer no great harm.
>>>>>
>>>>>
>>>>>
>>>>> I cannot offer the guarantee you seek in (3), but I don't think you
>>>>> want it. (If nothing changes, then the feature has no effect!) Here is a
>>>>> scenario where (3) could cause trouble:
>>>>>
>>>>>
>>>>>
>>>>> import Data.Set as Set ( abcde )
>>>>>
>>>>>
>>>>>
>>>>> data Set = Mk { abcdf :: Int }
>>>>>
>>>>>
>>>>>
>>>>> blah = Set.abcdf
>>>>>
>>>>>
>>>>>
>>>>> Previously, GHC would have suggested that you perhaps misspelled
>>>>> abcde. Now, you'll get (presumably) a type error.
>>>>>
>>>>>
>>>>>
>>>>> Here's another case:
>>>>>
>>>>>
>>>>>
>>>>> import Data.Set as Set ( Set )
>>>>>
>>>>>
>>>>>
>>>>> data Set = Mk
>>>>>
>>>>>
>>>>>
>>>>> x :: Set.Set
>>>>>
>>>>>
>>>>>
>>>>> Everything is happy today, but with -XLocalModules (and (3)), the type
>>>>> of x is an ambiguous name.
>>>>>
>>>>>
>>>>>
>>>>> Any example that causes trouble, though, will have something in
>>>>> common: an imported module name (possibly via an alias) that matches a
>>>>> locally defined type name. I would imagine this pattern is rare in
>>>>> practice, and that the benefit of (3) would outweigh the number of times
>>>>> that a problem like this bites.
>>>>>
>>>>>
>>>>>
>>>>> I, too, could live without (2).
>>>>>
>>>>>
>>>>>
>>>>> Richard
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>> _______________________________________________
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>>>> https://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-steering-committee
>>>>
>>>
>>
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