[ghc-steering-committee] #287: Simplify subsumption , recommendation: accept

[Iavor Diatchki]

I am in support

On Fri, Jan 17, 2020, 05:50 Spiwack, Arnaud <arnaud.spiwack at tweag.io> wrote:

> Dear all,
>
> As the shepherd for proposal #287 [
> https://github.com/ghc-proposals/ghc-proposals/pull/287 ], I recommend
> acceptance
>
> Summary
>
> The proposal is to remove a bunch of typing rules meant to make RankNTypes
> a bit more powerful (covariance and contravariance of the arrow
> constructor, as well as deep instantiation and deep skolemisation). The
> problem with these rules is that 1/ they complicate the type checker 2/
> They are implemented by performing eta-expansion during elaboration, which
> changes the semantics of program 3/ Dropping (some of) them is beneficial
> for the quick-look story (proposal currently on hold).
>
> The authors have tried this change on all of Stackage, and could come up
> with simple fixes (~300 lines of which) for all but 2 packages (namely,
> singleton, because of template haskell stuff, and labels for reason not
> well identified).
>
> Rationale
>
> The common point of all these features is that they jiggle the position of
> foralls around. Which sounds reasonable at first glance. But really, they
> tend to interact badly with other stuff (I don't remember the
> circumstances, but there was a discussion of deep instantiation and deep
> skolemisation in the context of linear types as well). Basically, they are
> rather fragile features, I'd rather we did without them.
>
> I think that 300 lines of simple fixes for all of Stackage is a very small
> number. So it is legitimate to consider this change to have low impact.
> Regarding the singleton, I'm sure Richard will be happy to fix it ;-) .
> Only labels remains. It shouldn't be a blocker.
>
> Remaining question
>
> There is one open question on the pull request thread. Namely, how to
> schedule the removal. Two paths are being discussed
>
>    1. Add a `-XLessSubsumption` flag, turn it off by default, then turn
>    it on by default in version n+1, then remove the subsumption rules
>    altogether in version n+3
>    2. Simply remove the code, preferably early in the release cycle,
>    communicate profusely.
>
> The thread participants seem to lean towards (2). The reason being that
> this is a change where it is easy to write code compatible with previous
> versions of the compiler (everything which compiles after this change also
> compiles before this change). Also, there is a preference for removing the
> relevant code sooner rather than later. But I'd like the opinion of the
> committee.
>
> Best,
> Arnaud
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