Re: Exception handling within a compound statement?

[Michael Riedl]

Phil,

here a simple sample from one of my libraries in the hope it will help ...

PROCEDURE CardPow(X : REAL;
                  N : CARDINAL) : REAL;

          VAR z : REAL;
BEGIN
      z:=1.0;
      LOOP
        IF ODD(N) THEN z:=z*X END; (* can give an overflow exception *)
        N:=N DIV 2;
        IF (N = 0) THEN EXIT; END;
        X:=X*X;                    (* can give an overflow exception *)
      END; (* LOOP *)
      RETURN z;
EXCEPT
      Errors.ErrOut("Overflow (LMathLib.CardPow)");
      RETURN INF;
END CardPow;

Errors.ErrOut is a simple routine displaying an error message on the 
standard error channel

and INF a predefined constant indicating IEEE positive infinity. The 
code is ISO M2 - PIM has no such features.

Gruß

Michael

Am 20.12.23 um 14:24 schrieb Gaius Mulley:
> "Fischlin  Andreas" <andreas.fischlin@env.ethz.ch> writes:
>
> > Dear Phil,
> >
> > Modula-2 has the philosophy of handling errors explicitly as the programer 
> > strives to write an error free code in the first place. The only exceptions 
> > that can
> > be considered are numerical ones, such as an integer or real overflow etc., 
> > which are detected and handled by the underlying hardware and system settings.
> > Depending on the current setting for the handling of such exceptions (tolerate 
> > or respond etc.) this results typically in a HALT of the Modula-2 program with
> > some information on the causes of the exception and/or triggering of a debugger.
> >
> > Hope this helps.
> >
> > Sincerely yours,
> > Andreas Fischlin
> >
> > ETH Zurich
> > Prof. em. Dr. Andreas Fischlin
> > Systems Ecology - Institute of Biogeochemistry and Pollutant Dynamics
> > CHN E 24
> > Universitaetstrasse 16
> > 8092 Zurich
> > SWITZERLAND
> >
> > andreas.fischlin@env.ethz.ch
> > https://sysecol.ethz.ch/people/andreas.fischlin.html
> >
> > +41 44 633-6090 phone
> > +41 79 595-4050 mobile
> >
> >               Make it as simple as possible, but distrust it!
> > ________________________________________________________________________
> >
> >   On 20 Dec 2023, at 04:20, Philip Munts <phil@munts.net> wrote:
> >
> >   Ada allows exception handling within each begin/end block, and
> >   also allows a begin/end block inside a compound statement:
> >
> >   loop
> >    begin
> >      raise Program_Error;
> >    exception
> >      when Program_Error =>
> >        ...
> >    end;
> >   end loop;
> >
> >   Does GM2 allow something similar, or can exceptions be caught
> >   only at the procedure or module level?
> >
> >   I like to use exceptions to propagate errors in Ada embedded
> >   real-time software, since they can be caught and handled
> >   anywhere along the calling chain or just ignored to abort the
> >   program.
> >
> >   If exceptions can only be caught at procedure or module level
> >   on the way out to a program abort, they are much less useful
> >   to me.
> >
> >   Phil
> Hi Andreas and Phil,
>
> very true for pim[234] (I guess on some systems nil violation might be
> caught by the underlying platform?).  In iso exceptions are allowed but
> only in the begin end blocks of modules and procedures.
>
> An interesting exception example from the iso standard can be found in:
>
>     gcc/testsuite/gm2/iso/run/pass/except8.mod
>
>     
> http://cvs.savannah.nongnu.org/viewvc/gm2/testsuite/gm2/iso/run/pass/except8.mod?view=markup
>
> (not that I'm advocating writing code like this :-).  On a more
> practical note the exception handling scheme works well and across
> languages - and if using swig across into scripting languages. (Python3
> can catch a m2 exception).  In gm2 the procedure THROW is available from
> the SYSTEM module and can be invoked from anywhere,
>
> regards,
> Gaius