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

[Eric Seidel]

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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