Re: [Caml-list] Labelled parameter bug?

[Philippe Veber <philippe.veber@gmail.com>]

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Hello,

2011/8/24 Dmitry Bely <dmitry.bely@gmail.com>

> 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 *)
>

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.



>
> Outputs:
>
> p1=p1, p2=p2, p3=p3 (1)
> p1=p1, p2=p3, p3=p2 (2)
>
> Why (1) and (2) are different? I assume f "p2" takes p3 instead of p2
> but then the compiler should issue at least a warning...
>
> - Dmitry Bely
>
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