[Haskell-cafe] out-commented code vs. case with redundant pattern matches
Sylvain Henry
sylvain at haskus.fr
Wed Nov 29 12:13:21 UTC 2017
> 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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