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From: brownc@utah-cs.UUCP
Newsgroups: net.lang.mod2
Subject: Exceptions in Modula-2
Message-ID: <3661@utah-cs.UUCP>
Date: Sat, 1-Feb-86 01:39:12 EST
Article-I.D.: utah-cs.3661
Posted: Sat Feb  1 01:39:12 1986
Date-Received: Sun, 2-Feb-86 01:05:00 EST
Reply-To: brownc@utah-cs.UUCP (Eric C. Brown)
Organization: University of Utah, Salt Lake City
Lines: 71

This is a message from a friend of mine; I *DID NOT* write this.  However,
I *will* forward responses to him.
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There have been several proposals to extend Modula 2 to include exceptions.
I believe that extending the language is not needed.  Below is a small
definition module that I think will take the place of the exceptions
proposed.  The implementation module should not be difficult to write.
Most of it can be stolen from the setjmp and longjmp code for C.  Of course
the implemenation module will be machine specific.

This code was inspired by Lisp's catch and throw.  For those who are not
familar with lisp catch takes a tag (any atom) and an expression to evaluate.
If in the process of evaluating the expression a throw is executed, the
system goes hunting for the catch with the same tag.  There is no
easy way to implement a tags in modula because each module may want to add
more tags.  There are several possible solutions:  use cardinals or integers
and have the user assure that the numbers are different between each different
case, or use strings.

Strings are attractive since they are much more descriptive that just a
number.  Notice in the definition module below there is no dependence on the
size of the string.  The implementation module may only use a limited size
string (e.g. only the first 32 chars are significant).



DEFINITION MODULE Exceptions;
EXPORT QUALIFIED CatchResult, Catch, Throw, Caught;

TYPE
	CatchResult = (NormalExit, Thrown);

PROCEDURE Catch(tag: ARRAY OF CHAR; p: PROC): CatchResult;

PROCEDURE Throw(tag, prompt: ARRAY OF CHAR);

PROCEDURE Caught(tag: ARRAY OF CHAR): BOOLEAN;

END Exceptions.



One other nice feature is the function Caught.  This allows the lower level
module to take care of the problem itself.  Example:

    IF Caught("DiskFull") THEN
	    Throw("DiskFull");
	ELSE
		WriteString("***** FATAL ERROR ***** The disk is full!");
		HALT;
	END; (* IF *)

I have not yet implemented the implementation module because I have not needed
exceptions.


I would like to some other discussion of proposed extentions to Modula 2.
In February's Computer Language there is a discussion of many extensions
to the language.  In my opinion most of them are not needed.  Any other
comments?

Jeffrey McArthur

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End of forwarded message.

Eric C. Brown

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