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@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@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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