Is simplified subsumption really necessary?

[John Ericson]

On 6/16/21 12:00 PM, Simon Peyton Jones via ghc-devs wrote:

> I’m sorry to hear that Chris. It’s exactly backwards from what I would 
> expect – the typing rules with simple subsumption are, well, simpler 
> than those for complicated subsumption, and so one might hope that 
> your intuition had fewer complexities to grapple with.
>
In https://richarde.dev/papers/2021/stability/stability.pdf it is written

    The analysis around stability in this paper strongly suggests that
    GHC should use the lazy, shallow approach to instantiation. Yet the
    struggles with lazy instantiation above remain. In order to simplify
    the implementation, GHC has recently (for GHC 9.0) switched to use
    exclusively eager instantiation.This choice sacrifices stability for
    convenience in implementation.

I think the principles outlined in the paper are very good, and explain 
the queasiness some users may feel in 9.0

> But wouldn't it be possible to choose a desugaring with seq that 
> doesn't do so?
>
> I just don’t know how to do that.  Maybe someone else does.
>
Is it not

   f `seq` \x -> f x

and similar? I haven't thought about the issue in a while or in very 
much depth, but when I first discussed the proposal years back with some 
other people at work, they spit-balled the same counter-proposal.

----

Having little "skin in the game" as I haven't yet ported any serious 
programs over to 9.0, I suppose I am glad the experimentation with 
QuickLook is happening, and OK that our accepting on-par fewer programs 
now opens design space for later (i.e. we got the breakage out of the 
way.) But I certainly think there are improvements in the spirit 
outlined in Richard's paper to be done down the road.

John

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