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From: bobc@tikal.UUCP
Newsgroups: comp.lang.modula2
Subject: Re: Exception handling anyone?
Message-ID: <623@tikal.UUCP>
Date: Wed, 11-Feb-87 13:56:09 EST
Article-I.D.: tikal.623
Posted: Wed Feb 11 13:56:09 1987
Date-Received: Sat, 14-Feb-87 04:37:13 EST
References: <354@esunix.UUCP>
Reply-To: bobc@tikal.UUCP (Bob Campbell)
Organization: Teltone Corp., Kirkland, WA
Lines: 124

Summary:

I would like to see a much more involved discussion on how to
implement exceptions in the language as it exists now.  I feel that
something useful can be worked out.  So I hope to inspire some
creative discussion by submitting the following exception handling
module.  I have not yet had the chance to implement it, but I feel
that all but the last three routines can be implemented in the
language as it exists now, with out adding any new features.  The last
three routines are mostly the "C" setjmp/longjmp, or the LISP
catch/throw, I used the LISP names to set them apart from the "C"
routines.

I believe that about a year ago someone else proposed something of
this form, but I can't find the article.  I also admit that there are
some advantages to implementing the constructs in the language it's
self.  They mostly relate to scoping for example:

    With my module you could write:

	IMPORT StringOverFlow;  (* An EXCEPTION defined and set up else where *)
	...
	BEGIN
	    ...
	    Handle(StringOverFlow,MyHandler);
		...call some string routines that use StringOverFlow...
	    FreeHandler(StringOverFlow);
	    ...

    With a Ada you could write:

    -- StringOverFlow is defined else where and imported USE/WITH???

    begin
	...call some strings routines that use StringOverFlow...
    exception
	when StringOverFlow =>
	    if not MyHandler(StringOverFlow) then
		raise;
	    end if;
    end ...;

I feel that both implementations do the same work, however the Ada
method does many things automaticly for you based on the nesting
factor.  It in effect automaticly calls FreeHandler when you exit the
current block.

This note is getting a little bit long, so I will stop and let people
flame at me as per usual.  Then when I know what the complaints are I
will correct/defend myself.

Bob Campbell
Teltone Corporation		18520 - 66th AVE NE
P.O. Box 657			Seattle, WA 98155
Kirkland, WA 98033

bobc@tikal.teltone.com
{amc,dataio,fluke,hplsla,sunup,uw-beaver}!tikal!bobc
tikal!bobc@beaver.cs.washington.EDU

Module Exceptions.DEF included below:
--------------------------------------------------------------
DEFINITION MODULE Exceptions;

TYPE
    EXCEPTION;

    ExceptionHandler	=	PROCEDURE (EXCEPTION):BOOLEAN;
	(* A exception handler is passed the exception as a argument
	 * so that it can find out (via comparing) what exception was
	 * caught
	 *)

PROCEDURE NewException(VAR X:EXCEPTION;DefaultHandler:ExceptionHandler);
    (* Create a new exception with a default handler
     * Should a default handler be do nothing or halt?
     *)

(* Some optional Default handlers *)
PROCEDURE Ingore(VAR X:EXCEPTION):BOOLEAN;
    (* always returns true causes exception to be ingored *)
PROCEDURE Abort(VAR X:EXCEPTION):BOOLEAN;
    (* always calls HALT *)

PROCEDURE Handle(X:EXCEPTION;Handler:ExceptionHandler);
    (* Set up a handler this pushes the previous handler onto a
     * Stack so that when FreeHandler is called it reverts to the
     * previous Handler.
     *)
PROCEDURE FreeHandler(X:EXCEPTION);
    (* frees up the current handler, if the current handler is the
     * default handler nothing happens.
     *)

PROCEDURE Raise(X:EXCEPTION);
    (* call the handlers for the passed exception.  The handlers are
     * called in reverise order until one of the handlers return TRUE
     * to indicate that it has handled the exception.
     *)

(* Now onto the optional low level routines a type of setjmp/longjmp
 * as in C, but I desided to use the LISP function names Catch, and
 * Throw
 *)

TYPE
    Ball;	(* Lets play catch with the ball :-) :-) *)
		(* This might be too cute a name :-) :-) *)

PROCEDURE Catch(VAR what:Ball):BOOLEAN;
    (* This function returns TRUE when it is thrown to, FALSE
     * when it is first set up
     *)

PROCEDURE Throw(what:Ball);
    (* jumps to that last call to Catch for this "Ball" restores
     * that stack frame, and causes it to apear that Catch returned
     * true
     *)

PROCEDURE FreeCatcher(VAR what:Ball);
    (* sets up the Ball such that attempting to Throw it will cause
     * a halt
     *)

END Exceptions.