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From: Martin Klaiber <martinkl@zedat.fu-berlin.de>
Newsgroups: comp.lang.ada
Subject: Re: Error-names.
Date: Sat, 28 Feb 2004 19:31:56 +0100
Organization: Freie Universitaet Berlin
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Marius Amado Alves <amado.alves@netcabo.pt> wrote:

> First, embrace long names. The standard C expression is (I think) "error 
> code", so go on and just write Constraint_Error_Code, General_Error_Code, etc.

Hey, thanks, that sounds good.

> Second, I'm not sure I understand your tentative code

Well, in reality it's a library to calculate income tax, which is, of
course written in Ada but shall get a C-Interface now.  The 'problem'
is that I have loads of procedures and functions to set and get the
parameters (just counted, it's 35), but not all of them are valid for
every year, etc.  So I need return-values for the C-Interface, as C
doesn't understand the Ada-exceptions.

> but maybe I understand what you want. You want to handle all
> exceptions in one place, the handling being returning the
> corresponding error code.

Yes, because I have about 5 or more exceptions which I want to convert
into a return-value and everyone of the 35 functions needs this.  I am
not concerned about the writing but it's easy to make mistakes (later
for instance, when new code is added) if so much code is just repeated.

So, what I have now is like this:

    function Set_Parameter_A (A : A_Type) return C_Integer is
    begin
       ... -- set the parameter
       return 0; -- if the setting was ok.
    exception
       -- if the parameter is not defined for this year:
       when Not_Defined => return Not_Defined_Error;
       -- if the parameter is out of range which is also year-dependent:
       when Out_Of_Range => return Out_Of_Range_Error;
       -- if something else went wrong setting the parameter:
       when Constraint_Error => return Constraint_Error_Code;
       ... and so on.
       when others => return General_Error;
    end Set_Parameter_A;

Well, and the same for Set_Parameter_B, and so on, 35 times :-(
Ok, I could just copy and paste but I don't like the idea really.

Well, in reality the functions have names like Set_ALTER1, Set_HINZUR
or Set_RE4 but this should not be important here.  The obscure names
are the official names from the tax-laws.

> Well, the place has to be in a function that contains all the calls that may 
> raise the exceptions:

> function Main return Error_Code_Type is
>  -- exception-raising calls here
> exception
>  when E : others => return Error_Code_Of (E);
> end;

> Now you only have to implement Error_Code_Of.

> (Ain't top-down design great? ;-)

> The 'trick' you could be missing is the "E : others" construct above. E is 
> the exception occurence. From it you can go to the exception name. See 
> package Ada.Exceptions--or, better yet, section 11.4 of the ARM.

Well, if I understand it right, I still have to distinguish between
the different exceptions before I call Error_Code_Of, is this right?

Like this (assume I have 5 exceptions A to E):

  function Main return Error_Code_Type is
   -- exception-raising calls here
  exception
   when A : others => return Error_Code_Of (A);
   when B : others => return Error_Code_Of (B);
   when C : others => return Error_Code_Of (C);
   when D : others => return Error_Code_Of (D);
   when E : others => return Error_Code_Of (E);
  end;

But now, how does, for instance, Set_Parameter_A does get the return
value from Main?  Calling

    function Set_Parameter_A...
    begin
       ...
    exception
       when others => return Main;
       raise;
    end Set...

doesn't work, as I already mentioned.

Martin