Re[2]: Case split uncovered patterns in warnings or not?

I agree in principle, but then what about data types with strict fields? E.g. data SMaybe a = SNothing | SJust !a f :: SMaybe Bool -> () f SNothing = () Today, we'd suggest `SJust _`. But the checker can't differentiate between evaluation done by a pattern-match of the user vs. something like a strict field that was unlifted to begin with. So we'd suggest `SJust True` and `SJust False`. Similarly, we'd case split unlifted data types by default, but not lifted data types. I think I can easily make the whole function (`GHC.HsToCore.Pmc.Solver.generateInhabitingPatterns`) dependent on whether it's called from an EmptyCase or not, to recover the behavior pre-8.10. But actually I had hoped we can come up with something more general and less ad-hoc than the behavior of 8.8. Maybe there isn't and 8.8 already lived in the sweet spot. ------ Originalnachricht ------ Von: "Richard Eisenberg" An: "Sebastian Graf" Cc: "ghc-devs" Gesendet: 10.11.2021 04:44:50 Betreff: Re: Case split uncovered patterns in warnings or not?

Maybe the answer should depend on whether the scrutinee has already been forced. The new output ("We now say", below) offers up patterns that will change the strictness behavior of the code. The old output did not.

Reading the link below, I see that, previously, there was an inconsistency with -XEmptyCase, which *did* unroll one level of constructor. But maybe that made sense because -XEmptyCase is strict (unlike normal case).

I'm just saying this because I think listing the constructors in the -XEmptyCase case is a good practice, but otherwise I think they're clutterful... and strictness is a perhaps more principled way of making this distinction.

Richard

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On Nov 9, 2021, at 8:17 AM, Sebastian Graf wrote:

Hi Devs,

In https://gitlab.haskell.org/ghc/ghc/-/issues/20642 we saw that GHC

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= 8.10 outputs pattern match warnings a little differently than it used to. Example from there:

{-# OPTIONS_GHC -Wincomplete-uni-patterns #-}foo::[a]->[a]foo[]=[]fooxs=yswhere(_,ys@(_:_))=splitAt0xsmain::IO()main=putStrLn$foo$"Hello, coverage checker!" Instead of saying

ListPair.hs:7:3: warning: [-Wincomplete-uni-patterns] Pattern match(es) are non-exhaustive In a pattern binding: Patterns not matched: (_, []) We now say

ListPair.hs:7:3: warning: [-Wincomplete-uni-patterns] Pattern match(es) are non-exhaustive In a pattern binding: Patterns of type ‘([a], [a])’ not matched: ([], []) ((_:_), []) E.g., newer versions do (one) case split on pattern variables that haven't even been scrutinised by the pattern match. That amounts to quantitatively more pattern suggestions and for each variable a list of constructors that could be matched on. The motivation for the change is outlined in https://gitlab.haskell.org/ghc/ghc/-/issues/20642#note_390110, but I could easily be swayed not to do the case split. Which arguably is less surprising, as Andreas Abel points out.

Considering the other examples from my post, which would you prefer?

Cheers, Sebastian _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs