[Haskell-cafe] ghc should be able to deduce correct use of partial functions and not give a warning in such cases

[Andrew Lelechenko]

Hi George,

If you do not want to pattern match on `ys`, the simplest solution is to do what the GHC warning suggests you to do and use `drop 1` instead of `tail`. Another alternative is `Data.List.uncons` followed by `Data.Maybe.maybe`. 

Best regards,
Andrew

> On 20 Apr 2024, at 13:28, George Colpitts <george.colpitts at gmail.com> wrote:
> 
> Thanks for everybody's replies
> 
> To give more context the original code from Ch. 12 of Bird's Thinking Functionally with Haskell was:
> 
> let (xs, ys) = break (== ch) str in
>          if null ys then [] else [(xs, tail ys)])
> 
> I replaced it with 
> 
> let (xs, ys@(_ : t)) = break (== ch) str in
>          if null ys then [] else [(xs, t)])
> 
> On Tue, Apr 16, 2024 at 10:51 AM George Colpitts <george.colpitts at gmail.com <mailto:george.colpitts at gmail.com>> 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? 
>> 
>> Cheers
>> George
>> 
>> 
>> 
>> 
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