Pattern matching desugaring regression? Re: [Haskell-cafe] Why does my module take so long to compile?

[Troels Henriksen]

It is very likely that issue 17386 is the issue.  With

{-# OPTIONS_GHC -Wno-overlapping-patterns -Wno-incomplete-patterns
-Wno-incomplete-uni-patterns -Wno-incomplete-record-updates #-}

my module(s) compile very quickly.  I'll wait and see if GHC 9 does
better before I try to create a smaller case (and now I at least have a
workaround).

Sebastian Graf <sgraf1337 at gmail.com> writes:

> Hi,
>
> I'm not sure I see all the context of the conversation, but it is entirely
> possible that code with many local constraints regresses the pattern-match
> checker (which is accounted to Desugaring in the profile emitted by -v2),
> I'm afraid. That simply has to do with the fact that we now actually care
> about them, previously they were mostly discarded.
>
> I'd be glad if you submitted a relatively isolated reproducer of what is
> fast with 8.8 and slow with 8.10 (even better 9.0).
> I hope that things have improved since we fixed
> https://gitlab.haskell.org/ghc/ghc/-/issues/17836, which is part of 9.0 but
> not of 8.10.
>
> Cheers,
> Sebastian
>
> Am Mo., 15. Feb. 2021 um 19:04 Uhr schrieb Troels Henriksen <
> athas at sigkill.dk>:
>
>> Carter Schonwald <carter.schonwald at gmail.com> writes:
>>
>> > Ccing ghc devs since that’s a better forum perhaps
>> > Crazy theory:
>> >
>> > this is a regression due the the partial changes to pattern matching
>> > coverage checking in 8.10 that finished / landed in ghc 9
>> >
>> > Why:
>> > Desugaring is when pattern/ case statement translation happens I think?
>> > And the only obvious “big thing” is that you have some huge , albeit sane
>> > for a compiler, pattern matching
>> >
>> > I’d first check if the new ghc 9 release doesn’t have that regression in
>> > build time that you experienced.  And if it does file a ticket.
>> >
>> > I may be totally wrong, but that seems like a decent likelihood !
>>
>> You may be right!  Another module that regressed is also mainly
>> characterised by large-but-not-insane case expressions:
>>
>> https://github.com/diku-dk/futhark/blob/d0839412bdd11884d75a1494dd5de5191833f39e/src/Futhark/Optimise/Simplify/Rules.hs
>>
>> I'll try to split these modules up a little bit (I should have done so a
>> while ago anyway) and maybe that will make the picture even clearer.
>>
>> --
>> \  Troels
>> /\ Henriksen
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-- 
\  Troels
/\ Henriksen