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

[Alejandro Serrano Mena]

 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> 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
>
>   http://www.joachim-breitner.de/
>
>
>
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