[ghc-steering-committee] #409: Exportable named defaults, Recommendation: Accept

Fri Aug 20 02:13:16 UTC 2021

I'm happy with this. Thanks!

Richard

> On Aug 19, 2021, at 9:25 AM, Eric Seidel <eric at seidel.io> wrote:
> 
> Dear Committee,
> 
> Simon PJ has requested (on GitHub) that we merge all aspects of this proposal into a single extension NamedDefaults. We already had agreement that the import behavior should not be guarded by any extension, so I take this to mean the following. 
> 
> The proposal will introduce a single new extension NamedDefaults that enables:
> 
> 1. The ‘default C (T1, T2, …)’ syntax that specifies the defaulted class. 
> 2. Exporting defaulting rules as discussed (explicitly, in the export list). 
> 
> Defaulting rules are always imported (implicitly), with no need to enable the NamedDefaults extension. 
> 
> Any objections?
> 
> Sent from my iPhone
> 
>> On Aug 14, 2021, at 12:58, Simon Peyton Jones <simonpj at microsoft.com> wrote:
>> 
>> 
>> Eric
>>  
>> I’m in support too – but I have added three small qns to the GitHub thread.
>> 
>> Simon
>>  
>> From: ghc-steering-committee <ghc-steering-committee-bounces at haskell.org> On Behalf Of Spiwack, Arnaud
>> Sent: 05 August 2021 07:43
>> To: Eric Seidel <eric at seidel.io>
>> Cc: ghc-steering-committee at haskell.org
>> Subject: Re: [ghc-steering-committee] #409: Exportable named defaults, Recommendation: Accept
>>  
>> I'm very supportive of this proposal. Thanks to everyone who participated in the discussion. Like Eric, I don't see any value to the ImportedDefault extension, and would rather we removed it.
>> 
>>  
>> 
>> On Tue, Aug 3, 2021 at 4:10 AM Eric Seidel <eric at seidel.io <mailto:eric at seidel.io>> wrote:
>> 
>> Committee,
>> 
>> Mario has updated the proposal following some discussion on GitHub around the question of implicit vs explicit export and import of default rules. The result is
>> 
>> 1. *Implicit import*: any and all forms of `import M` also import any defaulting rules exported by M, like type classes.
>> 
>> 2. *Explicit export*: defaulting rules must be explicitly exported like named things, mostly. The one exception is that 
>> 
>>     module M (module N) where { import N }
>> 
>> does not re-export any defaulting rules imported from N. Simon PJ argued strongly for this change on GitHub[1].
>> 
>> With that question settled, and with Simon and Richard's assent on GitHub, *I'd like to recommend that we accept the proposal*. However, I still do not see the need for a separate ImportedDefaults extension and would recommend that we enable the import behavior universally.
>> 
>> [1]: https://github.com/ghc-proposals/ghc-proposals/pull/409#issuecomment-882338794 <https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F409%23issuecomment-882338794&data=04%7C01%7Csimonpj%40microsoft.com%7C4209473397bb4a1e123d08d957dc5f41%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637637426303882159%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=QimksAfoQFVo25E0UgtDwLiOUFKzUgl24h5P%2BvO26Oc%3D&reserved=0>
>> 
>> On Mon, Jul 12, 2021, at 04:09, Simon Peyton Jones wrote:
>> >  
>> > I have added this as a comment in the GitHub repo, since others may 
>> > want to express an opinion
>> >  
>> > 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:* 11 July 2021 02:48
>> > *To:* Eric Seidel <eric at seidel.io <mailto:eric at seidel.io>>
>> > *Cc:* ghc-steering-committee at haskell.org <mailto:ghc-steering-committee at haskell.org>
>> > *Subject:* Re: [ghc-steering-committee] #409: Exportable named 
>> > defaults, Recommendation: Partial Accept
>> >  
>> >  
>> > 
>> > 
>> > > On Jul 9, 2021, at 12:35 AM, Eric Seidel <eric at seidel.io <mailto:eric at seidel.io>> wrote:
>> > >  
>> > > On Thu, Jul 1, 2021, at 13:16, Joachim Breitner wrote:
>> > > 
>> > >> a different way to phrase that question might be: Do we want these
>> > >> defaulting declarations to behave just exactly like named things, or
>> > >> exactly like typeclass instances, or do we afford a new class with it’s
>> > >> own exporting/importing behavior. Is that a fair assessment?
>> > > 
>> > > Not entirely, I think. 
>> > > 
>> > > We currently have two types of import/export behavior:
>> > > named things, and typeclass instances. The proposal as currently
>> > > written places defaulting rules somewhere in between: defaulting
>> > > rules are exported like named things, but imported like class instances.
>> > > This is new, but not too foreign, as the behavior on both sides exactly
>> > > matches existing behavior we're familiar with. It's just the combination
>> > > that's new.
>> >  
>> > This doesn't match my understanding of the proposal. It looks to me 
>> > that, as written in the proposal, exports of a `default` would have to 
>> > be explicit. That is, a module starting with `module M where ...` would 
>> > not export any defaults. This fact is a bit implied in the proposal 
>> > ("This proposal does not modify that behaviour: a `default` declaration 
>> > by itself does not apply outside its module."), but it's my best 
>> > understanding. 
>> >  
>> > ---
>> >  
>> > Simon and I have discussed. We both came to an agreement that imports 
>> > should have to be explicit.
>> >  
>> > GHC currently has two import/export strategies.
>> >  
>> > Strategy 1: Always. In the Always strategy, an entity is always 
>> > exported from a module and always brought into scope from an imported 
>> > module. The Always strategy is used for type and class instances.
>> >  
>> > Strategy 2: Public. In the Public strategy, an entity is exported by 
>> > default (no export list) or when explicitly included in an export list. 
>> > It is brought into scope from an importing module by default (no import 
>> > list) or when explicitly included in an import list. A Public entity 
>> > may be excluded from scope by a `hiding` clause. All top-level named 
>> > entities are exported/imported via the Public strategy.
>> >  
>> > I propose (with Simon's support)
>> >  
>> > Strategy 3: Private. In the Private strategy. an entity is exported 
>> > only when explicitly included in an export list, and it is brought into 
>> > scope from an imported module only when explicitly included in the 
>> > import list. I propose we use Private for `default` declarations (only).
>> >  
>> > Reasons:
>> >  
>> > * Changing defaulting behavior really can launch the rockets. Suppose T 
>> > has a Num instance whose fromInteger uses unsafePerformIO to launch the 
>> > rockets. Then including T in an import list could make a very 
>> > innocent-looking `x = 5` declaration launch the rockets.
>> >  
>> > * GHC currently supports an option -ddump-minimal-imports, which 
>> > displays import lists describing what symbols must be brought into 
>> > scope from an imported module. If a `import M` import statement brought 
>> > defaulting behavior into scope, then going from `import M` to `import M 
>> > (foo, bar)` might deleteriously change defaulting behavior, thus 
>> > invalidating the work of -ddump-minimal-imports.
>> >  
>> > * The proposal as written does not describe how `module` exports work 
>> > with named defaults. For example, what happens in `module B (module A) 
>> > where import A`? Normally, that re-exports all names in scope both as 
>> > `A.blah` and as `blah`. But, of course, a default isn't named in this 
>> > way. So is the default exported? By requiring explicit inclusion in the 
>> > export list, the Private strategy sidesteps this question.
>> >  
>> > * This is a more conservative choice. We can always revisit this in the 
>> > light of experience. However, if defaults were always imported, it 
>> > would be much more disruptive to make them imported only by request.
>> >  
>> > We have rightly identified that using the Private strategy would 
>> > potentially reduce the usefulness of this idea, especially with 
>> > alternative Preludes. As far as I know, GHC does not currently 
>> > officially support having an alternative Prelude. That is, an 
>> > "alternative Prelude" is really just disabling the import of 
>> > base.Prelude and then importing some other module. However, we could 
>> > imagine a compiler flag that specifies another package (or module name) 
>> > to use as the Prelude... and then we could also specify how it is 
>> > imported. For example, we could say that the Prelude is imported with
>> >  
>> > > import Prelude
>> > > import Prelude ( default(..) )
>> >  
>> > where the second line says to grab all the defaults. I think this would 
>> > be reasonable, but not necessary in the first version of this current 
>> > proposal.
>> >  
>> > Richard
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