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

[Joachim Breitner]

How annoying, how can I now say “I’m trusting the shepherd’s
judgement” … ;-)

I guess in that case I’ll say I’m trusting Simon PJ, and will go with
his vote, as I have no strong opinion of my own.

Cheers,
Joachim


Am Sonntag, den 13.09.2020, 22:12 +0000 schrieb Richard Eisenberg:
> 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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-- 
Joachim Breitner
  mail at joachim-breitner.de
  http://www.joachim-breitner.de/