Christophe Raffalli wrote:
> 
> hello,
> 
> --------------------------------
> let f b l = match l with
>   [] | [_] -> 1
> | [a;_] | [_;a] when a = b-> 2
> | _ -> 3
> 
> let _ =
>   print_int (f 1 [1;2]);
>   print_int (f 1 [2;1]);
>   print_newline ()
> --------------------------------
> 
> Do you think this code should have a useless pattern warning and print
> "23" instead of "22" ?

Well, both happens, right?  I don't see any problem.

The pattern [_;a] is useless because [a;_] matches the same values and
the first match (left-to-right) policy is specificed
(http://caml.inria.fr/pub/docs/manual-ocaml/manual014.html).

The guard is checked after the binding, and there is no backtracking.
This explains why the function returns 3 when b=1 l=[2;1].

-- Alain