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
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