[Haskell-cafe] Redundant/overlapping patterns?

[Jeff Clites]

I think it’s saying it’s redundant because you already known what constructor it is; I’m guessing you’d get the same error from “case A of A -> …”.

Jeff

> On Jul 21, 2023, at 2:57 AM, Ben Franksen <ben.franksen at online.de> wrote:
> 
> So the error message should talk about "unreachable pattern" instead of "redundant pattern". That would cover all situations and would be less confusing in this special one.
> 
> Cheers
> Ben
> 
>> Am 20.07.23 um 06:53 schrieb Viktor Dukhovni:
>>> On Wed, Jul 19, 2023 at 09:24:20PM -0700, Todd Wilson wrote:
>>> Can someone please explain this:
>>> 
>>> ghci> case 1 of 2 -> 3
>>> 
>>> <interactive>:1:11: warning: [-Woverlapping-patterns]
>>>     Pattern match is redundant
>>>     In a case alternative: 2 -> ...
>>> *** Exception: <interactive>:1:1-16: Non-exhaustive patterns in case
>>> 
>>> The non-exhaustive patterns part is obvious, but what is redundant about
>>> this match? And how can there be overlapping patterns when there's only one?
>> The error message changed betweek 8.8 and 8.10:
>>     GHCi, version 8.8.4: https://www.haskell.org/ghc/  :? for help
>>     λ> data X = A | B
>>     λ> case A of B -> 42
>>     *** Exception: <interactive>:2:1-18: Non-exhaustive patterns in case
>>   vs.
>>     GHCi, version 8.10.5: https://www.haskell.org/ghc/  :? for help
>>     λ> data X = A | B
>>     λ> case A of B -> 42
>>     <interactive>:2:11: warning: [-Woverlapping-patterns]
>>         Pattern match is redundant
>>         In a case alternative: B -> ...
>>     *** Exception: <interactive>:2:1-17: Non-exhaustive patterns in case
>> The exception part is the same, and I hope non-controversial, the
>> pattern match is indeed non-exhaustive.  What's new is the compile-time
>> warning.  It is documented at, e.g.:
>>     https://ghc.gitlab.haskell.org/ghc/doc/users_guide/using-warnings.html#ghc-flag--Woverlapping-patterns
>> where it is explained that a pattern is considered "overlapping" when it
>> is unreachable, which is true in this case, though for reasons other
>> than redundancy wrt. a prior pattern.  And indeed we see the true
>> issue/criterion is "reachability":
>>    λ> case A of B | GHC.Exts.considerAccessible -> 42
>>     *** Exception: <interactive>:3:1-47: Non-exhaustive patterns in case
>> The same is of course seen if the warning is disabled:
>>     λ> :set -Wno-overlapping-patterns
>>     λ> case A of B -> 42
>>     *** Exception: <interactive>:6:1-17: Non-exhaustive patterns in case
>> The pattern match is redundant given the specific scrutinee.  Perhaps
>> non-reachability in this case could be reported via a different warning,
>> but we have what we have.
> 
> -- 
> I would rather have questions that cannot be answered, than answers that
> cannot be questioned.  -- Richard Feynman
> 
> 
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