Re: Unclear error message - please help

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Alfred Hilscher <SPAM@alfred-hilscher.de> writes:

> Hello,
> 
> I have a problem with some code I've written. The compiler (GNAT 3.15p
> on Windows) reports an error whereas I think it should not.
> 
> 
> Package spec:
> 
> package Pkg is
>   procedure Proc (Host : STRING; RC : out Integer; Identifier : in
> Integer := 0);
> 
>   procedure Proc (Host : STRING;
>                  Time_Response : out Integer; RC : out Integer;
>                  Time_Wait : Integer := 1000;
>                  Identifier : Integer := 0);
> end Pkg;
> 
> 
> Main program:
> 
> with Text_IO;
> with Ada.Command_Line;  use Ada.Command_Line;
> 
> with Pkg; use Pkg;
> 
> procedure PkgTest is
>   RC : Integer;
> begin
>   if Argument_Count /= 1
>   then
>     Text_IO.Put_Line ("call: PkgTest <ip>");
>     return;
>   end if;
>   
>   Proc (Argument (1), RC, 2507);
>   Text_IO.Put_Line ("RC=" & Integer'Image (RC));
> end PkgTest;
> 
> 
> The compiler says, that the procedure call is ambigous, but I think, it
> is clear, that "Proc" from line 2 should be used, as it has parameters
> (in, out, in) while "Proc" from line 4 has (in, out, out). And the value
> (2507) in the call of "Proc" can not be used as an "out" parameter. It
> this a know error, or do I have strong misunderstandings there?
> 
> pkgtest.adb:15:03: ambiguous expression (cannot resolve "Proc")
> pkgtest.adb:15:03: possible interpretation at pkg.ads:4
> pkgtest.adb:15:03: possible interpretation at pkg.ads:2

As others have said, the compiler is correct.

I think that's a good thing.  The overload resolution rules should not
be "too smart" -- they should allow things to be ambiguous (and
therefore illegal) if there's a likelihood that the reader of the code
will be confused.

I think the call "Proc (Argument (1), RC, 2507);" is confusing, because
it's hard to know which Proc is called.  So it's good that it's illegal.
(It's illegal because parameter modes are not taken into account for
overload resolution.)  It's especially confusing because of all the
defaulted parameters.

When you have two or more parameters of the same type, named notation is
usually a good idea, as somebody suggested.  Otherwise, you might make a
mistake, and pass RC as the actual parameter for the Time_Response
parameter.

Other ideas:

Don't use Integer all over the place, but instead use
different types, to avoid confusion.

Or use two different names, instead of calling both "Proc".

- Bob