Wouldn't you have to be able to inline null and tail to be able to say that this use of tail is not partial? If null and tail "live" in a different library, is there much that you can do?

Thanks,

Ivan

On Tue, 16 Apr 2024 at 07:58, George Colpitts <george.colpitts@gmail.com> wrote:
Hi Henning,

Thanks for the quick response!
Yes, that's basically how I fixed it but I really don't want to have to do that since the code is correct. Not a big deal but may be irritating to beginners or large projects that want to eliminate warnings. Do you think an ER would be rejected ? If I remember correctly there is already detection of incomplete pattern matching that is ok and in such cases warnings are omitted. This would be similar to that.

Cheers,
George


On Tue, Apr 16, 2024 at 11:46 AM Henning Thielemann <lemming@henning-thielemann.de> wrote:

On Tue, 16 Apr 2024, George Colpitts wrote:

> ghc should be able to deduce correct use of partial functions and not give a warning in such cases, e.g.
> in 9.8.2
>
>      if null ys then [] else [(xs,tail ys)])
>
> gets a warning
>
>       warning: [GHC-63394] [-Wx-partial]
>
> but it is clear that this use of tail will never be a problem so IMHO that line of code should not get a
> warning. 
>
> Does anybody know if there is a plan or enhancement request to eliminate such warnings? 


I would just use good old pattern matching:

case ys of
    [] -> []
    _:zs -> [(xs,zs)]
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