[ghc-steering-committee] #283: Local modules (again), recommendation: accept
Simon Peyton Jones
simonpj at microsoft.com
Mon Jul 26 21:57:21 UTC 2021
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:
Not so great in cases where
import Graphics
imports Button.open and Widget.open
If you qualify, thus
import Graphics as G
then you want G.Button.open and G.Widget.open. You really don't want to strip off the carefully added Button and Widget qualifiers.
To be more explicit, I guess you could say
import Graphics( Buttton.*, Widget.* )
or perhaps more Haskell-y
import Graphics( module Button, module Widget )
and indeed I believe the proposal supports precisely that.
Currently, if I see an identifier "M.x", I can search for "M" and be sure to find it in the import list somewhere, either as "import M" or "import X as M".
Yes, that's convenient. But it would also be convenient if, which I see an identifier "x", I could always see where it is imported from. But sadly I can't - and I often find myself guessing. So it's not a new shortcoming of this proposal.
One could imagine a new flag, ExplicitBinders, whose principle is that it is easy to determine from every occurrence which import(s) brings it into scope. For an unqualified "x" that would mean you'd have to
* either import "x" explicitly, import M(x)
* or change the occurrence to a qualified M.x
That might be a good software engineering practice to follow. Then you could extend the same treatment to this new proposal.
Simon
From: Richard Eisenberg <rae at richarde.dev>
Sent: 26 July 2021 18:08
To: Simon Marlow <marlowsd at gmail.com>
Cc: Spiwack, Arnaud <arnaud.spiwack at tweag.io>; Simon Peyton Jones <simonpj at microsoft.com>; GHC Steering Committee <ghc-steering-committee at haskell.org>
Subject: Re: [ghc-steering-committee] #283: Local modules (again), recommendation: accept
On Jul 24, 2021, at 7:47 AM, Simon Marlow <marlowsd at gmail.com<mailto:marlowsd at gmail.com>> wrote:
OK I finished reading it again. Overall I like it because it's just name resolution; this is entirely in keeping with Haskell as it stands, and limits the interaction with current and future language features. As a smooth extension of the language I think it works nicely.
I rather like the idea of "data qualified" and "class qualified", perhaps because similar ideas work out quite well in other languages (e.g. C++).
Just one thing concerns me. I don't think this is necessarily a blocker, but it makes me vaguely uneasy.
Currently, if I see an identifier "M.x", I can search for "M" and be sure to find it in the import list somewhere, either as "import M" or "import X as M". I suspect a lot of people rely on this, and some coding styles strongly encourage the use of qualified names for this reason. Exporting qualified names and unfortunately breaks this property - any import can bring into scope qualified names.
The property could be recovered by requiring the explicit import of qualified names (import Data.Set (module Set, ..) or something like that) but that doesn't seem very appealing either. Curious whether other people are worried about this and/or have any suggestions.
This is true -- but it's simply a problem of unqualified imports. I can always have `import qualified Data.Set as S` and then use `S.Set`. The problem would be if `Data.Set` exports qualified names like `Set.fromList`. Now, if `Data.Set` is itself imported qualified, we have `S.Set.fromList`, which is unfortunate.
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.
Richard
Cheers
Simon
On Fri, 23 Jul 2021 at 11:00, Simon Marlow <marlowsd at gmail.com<mailto: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<mailto: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<mailto: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<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F283&data=04%7C01%7Csimonpj%40microsoft.com%7C56b2582ba6ca474db97e08d95057fa36%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637629161116302432%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=MqRkEYVSlrFk30MPbsmHX2Drf2R8IJocTDtRjZvUgIs%3D&reserved=0>
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<mailto: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<mailto:arnaud.spiwack at tweag.io>>
Cc: GHC Steering Committee <ghc-steering-committee at haskell.org<mailto: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<mailto: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<mailto:rae at richarde.dev>> wrote:
On May 26, 2021, at 3:28 AM, Spiwack, Arnaud <arnaud.spiwack at tweag.io<mailto: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<mailto:rae at richarde.dev>> wrote:
On May 25, 2021, at 3:09 PM, Alejandro Serrano Mena <trupill at gmail.com<mailto: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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