Case split uncovered patterns in warnings or not?

[Oleg Grenrus]

It should not. Not even when forced.

I have seen an `Integer` constructors presented to me, for example:

    module Ex where
   
    foo :: Bool -> Integer -> Integer
    foo True i = i

With GHC-8.8 the warning is good:

    % ghci-8.8.4 -Wall Ex.hs
    GHCi, version 8.8.4: https://www.haskell.org/ghc/  :? for help
    Loaded GHCi configuration from /home/phadej/.ghci
    [1 of 1] Compiling Ex               ( Ex.hs, interpreted )
   
    Ex.hs:4:1: warning: [-Wincomplete-patterns]
        Pattern match(es) are non-exhaustive
        In an equation for ‘foo’: Patterns not matched: False _
      |
    4 | foo True i = i
      | ^^^^^^^^^^^^^^

With GHC-8.10 is straight up awful.
I'm glad I don't have to explain it to any beginner,
or person who don't know how Integer is implemented.
(9.2 is about as bad too).

    % ghci-8.10.4 -Wall Ex.hs
    GHCi, version 8.10.4: https://www.haskell.org/ghc/  :? for help
    Loaded GHCi configuration from /home/phadej/.ghci
    [1 of 1] Compiling Ex               ( Ex.hs, interpreted )
   
    Ex.hs:4:1: warning: [-Wincomplete-patterns]
        Pattern match(es) are non-exhaustive
        In an equation for ‘foo’:
            Patterns not matched:
                False (integer-gmp-1.0.3.0:GHC.Integer.Type.S# _)
                False (integer-gmp-1.0.3.0:GHC.Integer.Type.Jp# _)
                False (integer-gmp-1.0.3.0:GHC.Integer.Type.Jn# _)
      |
    4 | foo True i = i
      | ^^^

- Oleg

On 9.11.2021 15.17, Sebastian Graf wrote:
> Hi Devs,
>
> In https://gitlab.haskell.org/ghc/ghc/-/issues/20642
> <https://gitlab.haskell.org/ghc/ghc/-/issues/20642> we saw that GHC >=
> 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 [] = []
> foo xs = ys
> where
> (_, ys@(_:_)) = splitAt 0 xs
>
> main :: 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
> <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 at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/ghc-devs/attachments/20211110/74ceeb0a/attachment.html>