Re: [ghc-steering-committee] Please review #319: NoFallibleDo proposal, Shepherd: Eric Seidel

17 Dec 2021

      To recapitulate, this proposal remarks that GHC does a
pre-pattern-matching-exhaustiveness check during the renaming phase to
figure out how whether it needs to insert fail in do-notation statements of
the form C x1… xn <- u. It notes that this precheck is heuristic, and
decides whether a MonadFail constraint will be required during type
checking. It argues that this is bad because you may end up being asked for
a MonadFail constraint when you were careful to write an exhaustive pattern
dependent on a GADT’s argument. And that it is a fundamental limitation of
the current desugaring scheme.

All these observations look correct (and were validated, in the PR’s
discussion by Sebastian Graf who is quite well-versed in the relevant parts
of the code). The argument that it is a bad thing is relatable (heuristic
in type checking are generally undesirable).

The proposal’s solution is to add an extension which would switch the
desugaring of C x1… xn <- u statements to never use fail (instead throw an
imprecise exception like regular case expressions). And have
incomplete-pattern-matching warnings. Basically treating <- like a let in
this context. I think.

The problem I see with this solution is that, while it can be argued that
it would have been desirable to do just that at the inception of Haskell,
it is quite unlikely that we can ever make this the default (considering
how much code exists that rely on the current fail-based desugaring, and
how hard it would be to track). So such an extension would remain as a
switch forever. And I don’t find that it’s particularly worth it.
------------------------------

John Ericsson, who is one of the co-author of the proposal, also wrote a
companion proposal https://github.com/ghc-proposals/ghc-proposals/pull/327
In this second proposal, he introduces a new statement form for the do
notation: case C x1 … xn <- u of { alts }, such that

do
  { case C x1 … xn <- u of { alts }
  ; stmts }

desugars to

do
  case u of
  { C x1 … xn -> do { stmts }
  ; alts }

The proposal is still marked as WIP, but the general idea is reasonable.
This is intended as a replacement of the fail desugaring, by inserting fail
(or whatever you see fit) manually: case C x1 … xn <- u of { _ -> fail }.

But, from my point of view, this proposal could just as well serve as a
replacement of the NoFailibleDo proposal being discussed in this thread.
Since you could force a fail-free pattern-matching as case C x1 … xn <- u
of {}.

I personally find this direction much more compelling.

/Arnaud

On Fri, Dec 17, 2021 at 10:48 AM Alejandro Serrano Mena 
wrote:
...
Dear all,
I missed it back then, but the authors of the “NoFallibleDo” proposal have
re-submitted to the Committee.
It seems though that we are still in some form of impasse, without leaning
towards acceptance or rejection. From the discussion, I think that the
feeling is that this is a desirable feature, but there are different
opinions about whether this should be per-module or per-block. It would be
great if all of us would discuss this matter (either here or in the GitHub
thread) and try to come to a conclusion (or ultimately cast a vote to
decide).
The proposal itself is about being able to tweak whether an incomplete
pattern match in a ‘do’ block generates a call to ‘fail’ — as it does now,
leading to an additional MonadFail constraint — or works as any other
pattern match — leading to a PatternMatchFail exception when a non-matching
value comes there.
Once again, I would love to hear your opinions :)
Regards,
Alejandro
El 23 jul 2021 13:40:26, Alejandro Serrano Mena 
escribió:
...
I’ve been made aware that the “NoFallibleDo” proposal has been
re-submitted to the Committee. My current recommendation is “reject”, as
outlined in the following comment
https://github.com/ghc-proposals/ghc-proposals/pull/319#issuecomment-8855800... (TL;DR,
you’d often like to enable this for a particular “do” block, not for an
entire file).
Regards,
Alejandro
El 28 jul 2020 11:33:02, Alejandro Serrano Mena 
escribió:
...
Done. Once again, sorry for the confusion.
Alejandro
El mar., 28 jul. 2020 a las 11:30, Simon Peyton Jones (<
simonpj@microsoft.com>) escribió:
...
OK, so to summarise
- We are waiting for the author
   - You are encouraging us to comment anyway
Correct?  Does the author know this?   Why encourage only us?   Maybe
post on Github to clarify the status, and encourage everyone to contribute.
S
*From:* Alejandro Serrano Mena 
*Sent:* 28 July 2020 10:25
*To:* Simon Peyton Jones 
*Cc:* ghc-steering-committee@haskell.org
*Subject:* Re: [ghc-steering-committee] Please review #319:
NoFallibleDo proposal, Shepherd: Eric Seidel
I mean the last status, push back to the author for revision.
Alejandro
El mar., 28 jul. 2020 a las 11:24, Simon Peyton Jones (<
simonpj@microsoft.com>) escribió:
So I’m still confused.  “We went back to GIthub”… does that mean that
we invited the author to revise and resubmit?  I don’t know what else “back
to github” means.
If it’s in committee-decision status, then our process says  should
either accept, reject, or push back to the author for revision, in a timely
way (guided by the shepherd)
Simon
*From:* Alejandro Serrano Mena 
*Sent:* 28 July 2020 10:22
*To:* Simon Peyton Jones 
*Cc:* ghc-steering-committee@haskell.org
*Subject:* Re: [ghc-steering-committee] Please review #319:
NoFallibleDo proposal, Shepherd: Eric Seidel
Eric was initially in charge, but I took over his duties. He thought
that a bit more discussion was needed, something I agree with, so we went
back to GitHub.
Sorry about the stale status, I feel that my back-and-forth was not
very clear.
Alejandro
El mar., 28 jul. 2020 a las 11:17, Simon Peyton Jones (<
simonpj@microsoft.com>) escribió:
Alejandro, this one hasn’t been on my radar.
Are you the shepherd?   Have you made a recommendation?  Is the
proposal in its final form  -- ie having absorbed all discussion etc?
Simon
*From:* ghc-steering-committee <
ghc-steering-committee-bounces@haskell.org> *On Behalf Of *Alejandro
Serrano Mena
*Sent:* 28 July 2020 09:22
*To:* Joachim Breitner 
*Cc:* ghc-steering-committee 
*Subject:* Re: [ghc-steering-committee] Please review #319:
NoFallibleDo proposal, Shepherd: Eric Seidel
Dear Committee,
I would like to kindly ask for your input in the NoFallibleDo proposal
-> https://github.com/ghc-proposals/ghc-proposals/pull/319
https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fghc-proposals%2Fghc-proposals%2Fpull%2F319&data=02%7C01%7Csimonpj%40microsoft.com%7C250fdb186a4a41772fe908d832d82b6c%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637315251305340206&sdata=DHvjx8qWwjNaa9jn1mdfyBBOqMLJAXdozi3otokRKbk%3D&reserved=0
This was submitted, then there was some discussion, but the
conversation has stalled.
Regards,
Alejandro
El jue., 14 may. 2020 a las 17:30, Alejandro Serrano Mena (<
trupill@gmail.com>) escribió:
@Eric congratulations! enjoy! :)
@Joachim I can take care of this, I think the direction Eric was
pushing this is a good one.
El jue., 14 may. 2020 a las 12:16, Joachim Breitner (<
mail@joachim-breitner.de>) escribió:
Hi,
Am Mittwoch, den 13.05.2020, 15:19 -0500 schrieb Eric Seidel:
...
My wife and I just checked into the hospital to have our second child
Congrats, and all the best!
...
, so I’m going to be short on time for committee duties for a few
weeks. I think it would be best to reassign this proposal so we don’t
keep the authors waiting.
Any volunteers?
Cheers,
Joachim
--
Joachim Breitner
  mail@joachim-breitner.de
  http://www.joachim-breitner.de/
https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.joachim-breitner.de%2F&data=02%7C01%7Csimonpj%40microsoft.com%7C250fdb186a4a41772fe908d832d82b6c%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637315251305340206&sdata=Xf0HrXmVOPpftJaGUUXQo2ztLCjz1sRhN4nJPP%2Fbshg%3D&reserved=0
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Re: [ghc-steering-committee] Please review #319: NoFallibleDo proposal, Shepherd: Eric Seidel

Spiwack, Arnaud