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

[Richard O'Keefe]

The clearest way that I know to write that code fragment is
  case break (== ch) str of
    (_,[]) -> []
    (xs,(_:ys)) -> [(xs,ys)]

It's one line longer, but there are no warnings and NO SURPRISES for the reader.
The version that matches on (_:_) before testing whether that's
possible surprised
me mightly.  Confused me, even.

On Sun, 21 Apr 2024 at 00:29, 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> 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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