[ghc-steering-committee] Proposal #274: Quicklook impredicativity, recommendation: accept

[Vitaly Bragilevsky]

Hi,

I support the proposal.

Vitaly

ср, 3 июн. 2020 г. в 12:04, Spiwack, Arnaud <arnaud.spiwack at tweag.io>:

> Dear all,
>
> Last time around that we had a discussion about this proposal, the authors
> asked to have some more time to revise a few things. Since then, the
> proposal has been simplified, and the paper accepted (congratulations!).
>
> Last time around, those of us who opined were mostly in agreement with
> accepting the proposal, though there were some distracting complications.
> Which have since been forked (and accepted) as proposal #287
> <https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-simplify-subsumption.rst>.
> I do believe that this iteration of the proposal is a no-brainer. It simply
> gives a clear semantics to the unspecified -XImpredicativeTypes. It’s
> useful, and an implementation already exists (!3220
> <https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3220>).
>
> The summary of the proposed specification is: to instantiate a type
> variable to something of the form forall a. …, go through arguments and
> look for an absolutely necessary such instantiation (which will happen when
> there is a data constructor: [a] ~ [forall b.t] iif a = forall b.t).
>
> The only point which we may want to discuss is ($), which currently has
> an ad hoc typing rule baked in the type checker. The proposal suggests
> removing this baked in rule, and replacing it with making ($) always use
> the -XImpredicativeTypes typing rule. An alternative, of course, is to
> simply not treat ($) any specially, and require -XImpredicativeTypes to
> be explicitly turned on when one wants runST $ do …. But, as the proposal
> points out, this would break a lot of existing programs. So I agree with
> the proposal’s choice.
>
> In summary, I recommend accepting this proposal in its current state.
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