Re: [Caml-list] Labelled parameter bug?

[Jacques Garrigue <garrigue@math.nagoya-u.ac.jp>]

On 2011/08/26, at 18:20, Dmitry Bely wrote:
> On Wed, Aug 24, 2011 at 7:01 PM, Philippe Veber
> <philippe.veber@gmail.com> wrote:
> 
>>> The following fragment compiles without a warning but produces strange
>>> results:
>>> 
>>> let f ?(p1="p1") ~p2 p3 =
>>>  Printf.printf "p1=%s, p2=%s, p3=%s\n" p1 p2 p3
>>> 
>>> let _ =
>>>  f "p2" "p3"; (* 1 *)
>>>  let f2 = f "p2" in
>>>  f2 "p3" (* 2 *)
> 
> (...)
> 
>> 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
> 
> Indeed. Thanks for the detailed explanation. But shouldn't the
> compiler decide that the partial application skipping labeled
> parameter is suspicious and generate a warning?

The golden rule here is no ambiguity.
Since this function accepts only two non-optional arguments (as the return type is unit),
if you pass them all at once without labels then this is accepted as a valid full application,
and cannot be confused with another interpretation.
Note that if you don't like this behaviour, you can manually activate warning 6 (Label omitted in function application)
which requires you to write all labels.

Jacques Garrigue