Yes, but that is an entirely different issue: See
https://gitlab.haskell.org/ghc/ghc/-/issues/13964,
https://gitlab.haskell.org/ghc/ghc/-/issues/20311 and my problems in
https://gitlab.haskell.org/ghc/ghc/-/merge_requests/4116#note_301577 and
following. Help is appreciated there, I don't know how to get the
necessary information in `DsM`. Would need to poke at `mi_exports`,
which is quite unreachable at that point. I'd probably have to add a
field to the `DsGblEnv`.
I agree that Integer is another nail in the coffin, but only by
coincidence. As I said in the issue, if you do an EmptyCase on `Integer`
(which you rarely should do), then you'd be presented with the abstract
constructors in GHC 8.8, too.
As for the issue at hand, I'll go for "case split on EmptyCase only",
which should get back the behavior from 8.8.
------ Originalnachricht ------
Von: "Vladislav Zavialov" <vladis...@serokell.io>
An: "Oleg Grenrus" <oleg.gren...@iki.fi>
Cc: "ghc-devs" <ghc-devs@haskell.org>
Gesendet: 10.11.2021 10:51:03
Betreff: Re: Case split uncovered patterns in warnings or not?
Integer is an interesting example. I think it reveals another issue:
exhaustiveness checking should account for abstract data types. If the
constructors are not exported, do not case split.
- Vlad
On 10 Nov 2021, at 12:48, Oleg Grenrus <oleg.gren...@iki.fi> wrote:
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 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, 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
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