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

[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