[ghc-steering-committee] Proposal #356: new syntax for linear arrows

[Alejandro Serrano Mena]

Hi,
I am OK with B or C, with a slight preference with B. I am OK too with
moving this proposal fast so it can be reflected on GHC 9.0.

Regards,
Alejandro

El lun., 14 sept. 2020 a las 0:12, Richard Eisenberg (<rae at richarde.dev>)
escribió:

> Hi committee,
>
> Mathieu Boespflug has proposed a new syntax for linear arrows:
> https://github.com/ghc-proposals/ghc-proposals/pull/356
>
> Currently (from the original linear types proposal), we have #-> to denote
> a linear function, and # m -> to denote an arbitrary function whose
> argument has multiplicity m. This proposal suggests three changes:
>
> 1. Drop #-> entirely.
> 2. Use ^ instead of # in the second form, but require that it be a
> *prefix* usage of ^ (that is, the character after the ^ must not be
> whitespace).
> 3. Introduce a new lexeme ^1 that is used to denote linear functions.
> Alternative: use % in place of ^; % is observed to be more visually
> apparent in some fonts than ^.
>
> In time, the hope is that rule (3) will no longer be needed, if we gain
> overloaded numbers at the type level (when 1 :: Multiplicity will
> type-check without a special rule).
>
> Motivations for this change come from the experience within Tweag of using
> linear types. The change away from # also is to avoid a conflict with
> overloaded labels, once those are allowed in types.
>
> The hope is that we can make a decision about this in time for release
> with GHC 9.0. This proposal is already implemented:
> https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4020
>
> In conflict with our usual protocol, I will not make a recommendation on
> this proposal, as I am conflicted with the proposer. However, I do hope
> that we can come to a conclusion on this quickly, to avoid making a
> backward-incompatible change after a release.
>
> We have four options:
> A. Keep the syntax as it is.
> B. Choose to go with ^
> C. Choose to go with %
> D. Choose to go with some other symbol. I don't think there is any
> particular preference for any one symbol over another.
>
> Richard
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