Path: utzoo!attcan!uunet!cs.utexas.edu!usc!elroy.jpl.nasa.gov!jpl-devvax!lwall
From: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Newsgroups: comp.lang.perl
Subject: Re: new menu.pl
Message-ID: <8923@jpl-devvax.JPL.NASA.GOV>
Date: 30 Jul 90 18:10:35 GMT
References: <1990Jul29.195437.7608@uvaarpa.Virginia.EDU>
Reply-To: lwall@jpl-devvax.JPL.NASA.GOV (Larry Wall)
Organization: Jet Propulsion Laboratory, Pasadena, CA
Lines: 80

In article <1990Jul29.195437.7608@uvaarpa.Virginia.EDU> marc@mit.edu writes:
: Of particular interest are the following lines:
: 
: 	sub sigint {
: 		$foo = "@_[0..]";  # always generates a perl error at runtime.
: 	}
: 
: 	# ...
: 
: 	# exception handling!
: 
: 	$oldsigint = $main'SIG{'INT'};
: 	$main'SIG{'INT'} = "menu'sigint";
: 
: 	eval("&$fct(\$arg);");
: 
: 	$main'SIG{'INT'} = $oldsigint;
: 
: I've basically implemented exception handling.  Since eval returns at
: any fatal error, the menu function can be aborted at any time by a
: perl error.  In this case, I install a signal handler which causes a
: perl error.  (Larry, is there any better way to generate a guaranteed
: perl error?)  It would be pretty trivial to implement throw- and
: catch-style semantics given this technique: (Following untested)

You don't need to get so fancy to generate a fatal error.  Just use the
die operator:

	sub sigint {
	    die 'Here is a parameter that will be passed back in $@' . "\n";
	}

: With some clever code, you could probably implement real catch and
: throw, with different kinds of throws and data.

Seems fairly easy to me.  Here's an implementation of typed exception
handlers:

sub catch {
    local($typelist, $code) = @_;
    local($retval);
    eval $code;
    if ($@ =~ /^(\w*):/) {
	$retval = $1;
	die $@ unless $typelist =~ /\b$retval\b/;	# propagate exception
	$@ =~ s/^\w*://;
    }
    elsif ($@ ne '') {
	die $@;
    }
    $retval;
}

sub throw {
    local($type, $message) = @_;
    die "$type:$message\n";
}

$except = &catch('INT MATH SYNTAX', '&calc');
if ($except eq 'MATH') {...}


sub calc {
    ...
    &throw('MATH', "Can't divide by zero") if $denom == 0;
    ...
}

Or something like that--I haven't tested it.

: I think this one can go under "functionality Larry never even dreamed
: about."

Don't teach your grandmother to suck eggs.   :-)

: Need another chapter in the book? :-)

Short chapter.  :-)

Larry