[ghc-steering-committee] #511 Deep Subsumption, recommendation: accept

[Spiwack, Arnaud]

Eric,

After the exchange on Github: are you still hesitant?

On Sun, Jun 26, 2022 at 3:24 PM Eric Seidel <eric at seidel.io> wrote:

> I think this is a particularly hard fork to stomach, because
> -XDeepSubsumption must be enabled in *client modules* in order to gain the
> usability benefits. That makes it quite infectious, and puts the burden of
> reasoning about complications from auto-eta-expansion on users rather than
> library authors.
>
> I've asked the authors on GitHub to consider an alternative where *library
> authors* flag modules/functions/parameters as being candidates for
> auto-eta-expansion instead. It seems like that may provide the same
> usability benefits to clients, but importantly puts the pebble in the right
> shoe wrt language complexity.
>
> On Tue, Jun 21, 2022, at 05:47, Simon Marlow wrote:
> > I'm never a fan of fork-like things, but as long as we're clear that
> > -XDeepSubsumption is not recommended and will not be on by default in a
> > future GHC20XX then I suppose it's OK.
> >
> > If it's backported to 9.2, then code wanting to use it would need to
> > have a `ghc >= 9.2.4` constraint in the `.cabal` file, which is a bit
> > unusual. We don't normally add new language features in a patchlevel
> > release. But this isn't a strong argument for not doing it I guess -
> > you would need that constraint if you relied on some bug that was fixed
> > in 9.2.4 too.
> >
> > Cheers
> > Simon
> >
> > On Mon, 20 Jun 2022 at 14:56, Spiwack, Arnaud <arnaud.spiwack at tweag.io>
> wrote:
> >> Dear all,
> >>
> >> The Deep Subsumption proposal [
> https://github.com/ghc-proposals/ghc-proposals/pull/511 ] proposes a new
> extension, -XDeepSubsumption, which, when activated, partially reverts the
> changes from the Simplified Subsumption proposal.
> >>
> >> The Simplified Subsumption breaks more programs than anticipated. Many
> don't see any benefit from Simplified Subsumption, just the breakage, and
> don't like the eta-expansion that it forces.
> >>
> >> -XDeepSubsumption, when activated, restores deep skolemisation and
> co/contra-variance of the function arrow (but not deep instantiation, which
> doesn't affect the observed breakage). The patch already exists for it, and
> is about 400loc.
> >>
> >> There are two interesting highlights for me.
> >> - It is proposed that -XDeepSubsumption is activated by default in
> Haskell98 and Haskell2010, but not GHC2021. -XDeepSubsumption is orthogonal
> to Haskell2010, as far as I can tell, but it gives a cut-off point from
> which the recommended behaviour (-XNoDeepSubsumption) is the default.
> >> - Even with -XDeepSubsumption, the Quick Look algorithm assumes that
> the function arrow is invariant. The consequences of that are difficult to
> anticipate, but there is no known example of a bad behaviour due to that
> interaction yet.
> >>
> >> The authors also have one unresolved question that I'm bringing to the
> committee's attention: should `-XDeepSubsumption` be backported to GHC 9.2?
> >>
> >> ---
> >>
> >> Despite the fact that this extension is decidedly fork-like, and that
> it's a real possibility to see the community split around this (after all,
> the motivation for -XDeepSubsumption is a few libraries which were designed
> to leverage GHC's deep subsumption, and may very well stay that way in the
> foreseeable future). I recommend acceptance. Providing a path to backward
> compatibility seems to me like the right thing to do.
> >>
> >> I also recommend backporting to GHC 9.2. It should essentially be
> backward compatible, and providing an update path that doesn't go directly
> from 9.0 to 9.4 feels better to me.
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> >>
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