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

[Eric Seidel]

I'm on board with acceptance. I do think a backport to older GHC 9.x's would make sense if it's not too much work

On Thu, Jun 30, 2022, at 09:50, Spiwack, Arnaud wrote:
> 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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