[ghc-steering-committee] #370: Syntax for Modifiers, Recommendation: Acceptance

[Richard Eisenberg]

To my surprise, I found myself leaning against. So I updated and simplified the proposal to remove Modifier. This makes modifiers a bit more magical, but more likely to actually work in practice. The type inference story previously may have been intractable.

I've requested that the committee consider the updates in parallel with community feedback.

Thanks,
Richard

> On Nov 30, 2020, at 2:36 PM, Alejandro Serrano Mena <trupill at gmail.com> wrote:
> 
> After some discussion in the GitHub thread, changes are going to arrive to the proposal. I think the best is to send the proposal back to the “Needs revision” state.
> 
> Regards,
> Alejandro
> 
> On 29 Nov 2020 at 23:12:44, Eric Seidel <eric at seidel.io <mailto:eric at seidel.io>> wrote:
> I left a few comments and questions on the PR itself, but I'm leaning towards rejecting the proposal in its current form as well. This doesn't (yet) feel like a generic mechanism, in particular because the only modifier that has been specified would be deeply wired into GHC itself.
> 
> On Fri, Nov 27, 2020, at 04:46, Joachim Breitner wrote:
>> Hi,
>> 
>> Am Donnerstag, den 26.11.2020, 14:58 -0500 schrieb Alejandro Serrano
>> Mena:
>> > Dear all,
>> > This proposal suggests adding syntax for a general notion of
>> > modifiers, like the ones we’ve been talking about lately affecting
>> > linearity or matchability of arrows. For example, if linear types and
>> > unsaturated families are accepted as they stand, we would have `Int
>> > #1 -> @U Bool` (or something like that), whereas with this proposal
>> > we would have the more uniform `Int %1 %Unmatchable -> Bool`.
>> > 
>> > Since the amount of modifiers is likely to increase in the future, I
>> > think it’s a great idea to agree and reserve such syntax, instead of
>> > coming up with different ways on each proposal. I thus recommend
>> > acceptance of this proposal.
>> > 
>> > The proposal itself:
>> > (1) introduces syntax for modifiers in types and defines how to
>> > type/kind check them,
>> > (2) reserved such syntax for other uses in declarations and terms.
>> > 
>> > I think the proposal still has its merits only with (1), even though
>> > I lean towards accepting both parts of it.
>> 
>> I like the idea of reserving syntax here, but parts of the proposal
>> smell a bit like premature generalization to me. Are we confident that
>> all annotations we eventually would like to use with this feature can
>> be expressed as types of a kind that is an instance of Modifier? Or
>> should we reserve the ability to have annotations that don't fit that
>> model?
>> 
>> Would we ever have annotation that may affect phases earlier than than
>> typechecking? What if we want to use (%type e) and (%data e) to help
>> with the SingleNamepace issues? Look like useful annotations to me, but
>> I am not sure if they fit the framework proposed here.
>> 
>> The fact that we special-case %1 supports that.
>> 
>> The proposal explicitly has to state “No modifier polymorphism!”. But
>> isn't that indication that using the type system to model the various
>> modifiers might be the wrong tool?
>> 
>> I wonder if there is a way where the %(…) on it’s own only reserve
>> syntax, and the various uses of that syntax can be disambiguated
>> _statically_ based on the content of …. 
>> 
>> Not great syntax, because not concise, enough, but morally I’d feel
>> more at ease with
>> 
>>   Int %(multiplicity Many) -> Int
>>   Int %(multiplicity 1) -> Int
>>   Int %(multiplicity m) -> Int
>> 
>> where multiplicity is a modifier keyword, to express the existing
>> features (including implicit generalization of m). Then we can extend
>> this to
>> 
>>   Int %oneShot -> Int
>> 
>> and
>> 
>>   Int %(matchability M) -> Int
>> 
>> and maybe even
>> 
>>   foo (%type [a]) -- where foo :: forall a -> ()
>> 
>> which is a modifier that 
>> 
>> 
>> So at the moment, I am inclined to reject this proposal, until I am
>> convinced that we are not painting ourselves into a “all modifiers are
>> types of special kinds and that’s all the syntax and behaviour we ever
>> need” corner.
>> 
>> Minor detail: If we can annotate infix use of the (->) “type operator”,
>> should we also be able to annotate other infix operators, i.e.
>> 
>>   constr ::= (btype | ! atype) {modifier} conop (btype | ! atype) 
>>   infixexp ::= lexp {modifier} qop infixexp
>> 
>> 
>> 
>> Cheers,
>> Joachim
>> 
>> 
>> -- 
>> Joachim Breitner
>>   mail at joachim-breitner.de <mailto:mail at joachim-breitner.de>
>>   http://www.joachim-breitner.de/ <http://www.joachim-breitner.de/>
>> 
>> 
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