Re: Exception handling

From: [at]} <Bob>
Date: Wed, 15 Sep 2004 19:20:55 +0100

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Content-type: text/plain

Hi Rene,

> Maybe I did not understand what you were saying, but I think of two
> ways currently used by the BB to do such a thing:

> 1. If you have a procedure, make it a "function", returning an integer
> as a return code. That way, the caller may decide how to react to a
> "grey" error.
> 2. Where you already return something valueable, have a VAR-parameter
> with the return code. If you must not complete your action due to the
> nature of the error, fill the variable, and then RETURN.

With an SCL-style WHENEVER clause you can turn something like this:

PROCEDURE P( ..., VAR res : INTEGER );
BEGIN
     P0(..., res );
     IF res = OK THEN
        P1(..., res );
        IF res = OK THEN
           P2( ..., res );
           IF res = OK THEN
              .
              .
              .
           END
        END
     END
END P;

into this:

PROCEDURE P( ..., VAR res: RESULT );
BEGIN
    WHENEVER res # OK DO
       some sort of recovery, or error message or whatever
       EXIT (* optional - you could continue with the procedure *)
    END;
    
    P0(..., res );
    P1(..., res );
    P2(..., res );
    .
    .
    .
END P;

with a function you could write:

PROCEDURE P( ... ): RESULT;
VAR res: RESULT;
BEGIN
     WHENEVER res # OK DO
        whatever
        RETURN res;
     END
   ...
   RETURN OK;
END P;

It is syntactic sugar, of course, but it can be very convenient and a
great aid to clarity. It also eliminates the daisy-chaining you can get
whether you use a RETURN or a VAR.

I've googled around a bit for some examples. They're quite hard to
find, but here's a page full of them:
http://www.mfleming.tele2.co.uk/vme/scl/

and here's one of them which gives a good flavour of it:
http://www.mfleming.tele2.co.uk/vme/scl/getcorr.txt

I always found it a very enjoyable and productive language to use.
When I first started using PCs I looked around for a language that was
similar. Somebody recommended Pascal or Modula-2, so I bought JPI Modula-2.
That was my introduction to the www (wonderful world of Wirth).

-- 
Regards,
 Bob
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Received on Wed Sep 15 2004 - 20:20:55 UTC