Is simplified subsumption really necessary?

[Simon Peyton Jones]

Richard's paper argues for lazy rather than eager instantiation.

It does not argue for deep rather than shallow subsumption and instantiation; on the contrary, it argues for shallow.  (That is, for "simple subsumption".)   And it is simple subsumption that is the focus of this conversation.

Simon

From: John Ericson <john.ericson at obsidian.systems>
Sent: 18 June 2021 16:56
To: ghc-devs <ghc-devs at haskell.org>
Cc: Simon Peyton Jones <simonpj at microsoft.com>
Subject: Re: Is simplified subsumption really necessary?


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<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fricharde.dev%2Fpapers%2F2021%2Fstability%2Fstability.pdf&data=04%7C01%7Csimonpj%40microsoft.com%7C7655b09d06a54a4af03508d9327193cd%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637596286139778988%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=R1quXKjsnEjusvX%2BSxhPT25t%2B%2Bwqwo7mTPhnulvctQ0%3D&reserved=0> 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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20210618/2111b6cd/attachment.html>