Re: Shortcutting a SAX parser

There are various ways to tweak how die works. IIRC it for one thing depends
on how you call it, with or without parenthesis and with or without an
argument. Also you can use various modules like Carp.pm to adjust how it
works, or install your own die handler. Finally there is no requirement that
the argument to die be any particular type of variable. All it has to be is
a scalar of some kind. It could be a string, number, hash ref, array ref,
maybe even a typeglob. Specifically it can be a blessed hash ref. So you can
throw objects out of a die. Anyway, you can definitely get a consistent
output string.

On Fri, Apr 3, 2009 at 12:04 PM, Michael Ludwig <[email protected]> wrote:

> Robin Berjon schrieb:
> > On Mar 31, 2009, at 06:33 , Michael Ludwig wrote:
>
> >> Is dying the way to go here? Or is there a more elegant and
> >> recommended way to terminate the parser? Other thoughts or comments?
> >
> > Yes, you guessed right (in fact it's documented in
> > http://perl-xml.sourceforge.net/perl-sax/ but I can't get you the
> > exact pointer as my hotel connection is horridly slow), that's a
> > perfectly valid approach.
>
> Hi Robin,
>
> thanks for answering! Here's the exact link, and the perldoc:
>
>   http://perl-xml.sourceforge.net/perl-sax/sax-2.1.html#Exceptions
>   perldoc XML::SAX::Exception
>
> > If you want to be absolutely strict and defensive in your programming,
> > you probably want to throw a specific exception (SAX exceptions are
> > expected to have at least a Message key with a reason for throwing)
> > and check that that's the one you're getting. That way, if your
> > document does turn out to have a problem, your code won't confuse
> > "threw because I got what I wanted" with "threw because something
> > smells bad".
>
> That's true, of course you really should do that. However, two things
> combine to make this difficult.
>
> First, after a die(), the $@ variable also contains (from the point of
> view of the program logic) random line number information, which, in a
> hackerish manner, you'd then have to weed out in order to compare the
> string to the expected one.
>
>   eval { die 'Gurke' };
>   print $@ if $@;
>
> That's not pretty. There must be a better way.
>
> Second, the documentation I referred to above does not describe the
> behaviour I'm observing. $@ is not a blessed hash reference, but a
> simple string. Maybe that has to do with the fact that the class
> overloads "", but then, I don't know how to get at the object. See
> test case below.
>
> Michael Ludwig
>
> milu@colinux:~/ds > expand -t2 chax.pl
> #!/usr/bin/perl
> # [comments snipped]
> package MySAXHandler;
> use strict;
> use warnings;
> use base 'XML::SAX::Base';
>
> @XML::SAX::Exception::Done::ISA = ('XML::SAX::Exception');
>
> sub start_element {
>   my( $self, $elm) = @_;
>    if ( $elm->{LocalName} eq $self->{elm_searched} ) {
>      $self->{callback}->( $elm);
>     throw XML::SAX::Exception::Done( Message => 'Doc done' );
>    }
> }
>
> sub new {
>   my( $pkg, %opt) = @_;
>   my $self = $pkg->SUPER::new( %opt);
>    $self->{elm_searched} ||= 'TVChannel'; # TVChannel Broadcast
>   return bless $self, $pkg;
> }
>
> # Hauptprogramm --------------------------------------------------------
> package main;
> use strict;
> use warnings;
> use XML::SAX;
> use Data::Dumper;
>
> my $callback = sub {
>   my $elm = shift;
>   my $att_id   = $elm->{Attributes}{'{}TVChannelID'};
>   my $att_name = $elm->{Attributes}{'{}TVChannelName'};
>   print $att_id->{Value}, "\t", $att_name->{Value}, "\n";
> };
>
> my %handler_opt = (
>   # elm_searched => 'Gurke',
>    callback => $callback,
> );
> my $handler = MySAXHandler->new( %handler_opt);
> my $parser = XML::SAX::ParserFactory->parser( Handler => $handler);
>
> while ( <> ) {
>   chomp;
>   eval { $parser->parse_uri( $_) };
>   print Dumper $@ if $@;
> }
>
> milu@colinux:~/ds > cat tv.xml
> <TVChannel TVChannelID="99" TVChannelName="ARD">
>         <Moin>Moin</Moin>
> </TVChannel>
>
> milu@colinux:~/ds > ls tv.xml | chax.pl
> 99      ARD
> $VAR1 = 'Doc done at /usr/local/lib/perl/5.8.8/XML/LibXML/SAX.pm line 73
>  at /home/milu/ds/chax.pl line 61
> ';
>
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-- 
The Wise adapts himself to the world. The Fool adapts the world to himself.
Therefore, all progress depends on the Fool.

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