The type of f is
val f : ?p1:string -> p2:string -> string -> unit = <fun>
so f "p2" applies to the only anonymous parameter (third one) because it cannot be applied to the first or second without label :
# f "p2";;
- : p2:string -> unit = <fun>
This first application also applies optional arguments situated before the anoymous argument, so it remains the second (labeled) argument only.
There is indeed a special case where you can drop labels if you provide the exact number of arguments. This means that f "p2" "p3" is equivalent to f ~p2:"p2" "p3". This is written in the manual (
http://caml.inria.fr/pub/docs/manual-ocaml/manual006.html) :
"As an exception to the above parameter matching rules, if an
application is total, labels may be
omitted. In practice, most
applications are total, so that labels can be
omitted in applications.
"
So this is actually the intended behavior, AFAIU
Philippe.