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.