[Haskell-cafe] out-commented code vs. case with redundant pattern matches

[Sylvain Henry]

 > I thought that "redundant" means that the first two cases overlap 
with '_'. But if I replace '_' by '2' I get not only the non-exhaustive 
patterns warning but an additional redundancy warning on pattern '2'.

I think it's a bug of the PM check instead: it doesn't correctly detect 
redundant Int alternatives ("1" and "_" in your example). Could you 
report it as a bug?
Note that it works as expected with Integer: "case 0 :: Integer of" 
reports "1 ->..." and "_ -> ..." as redundant.


A solution to avoid the warning is:

    let choosePut = 0 :: Int

    case choosePut of
       0 -> putStrLn "A"
       1 -> putStrLn "B"
       _ -> putStrLn "C"


-Sylvain


On 29/11/2017 11:04, Henning Thielemann wrote:
>
> Occasionally I have multiple implementations of the same task and want 
> to choose one quickly but statically. I do not want to out-comment 
> unused branches because they shall still be type checked. So far I 
> used this scheme:
>
>    case 0::Int of
>       0 -> putStrLn "A"
>       1 -> putStrLn "B"
>       _ -> putStrLn "C"
>
> With ghc-8.0.2 and ghc-8.2.2 I get these warnings:
>
> RedundantCase.hs:4:7: warning: [-Woverlapping-patterns]
>     Pattern match is redundant
>     In a case alternative: 0 -> ...
>
> RedundantCase.hs:5:7: warning: [-Woverlapping-patterns]
>     Pattern match is redundant
>     In a case alternative: 1 -> ...
>
> I thought that "redundant" means that the first two cases overlap with 
> '_'. But if I replace '_' by '2' I get not only the non-exhaustive 
> patterns warning but an additional redundancy warning on pattern '2'.
>
> Is there a nice way to tell GHC that the unused branches are intended, 
> without generally disabling overlapping patterns warning?
>
> I mean, this one does not provoke any warnings:
>
>    if True
>      then putStrLn "X"
>      else putStrLn "Y"
>
> but is limited to two branches.
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