WDDX.pm weird eval error

alienhuman has asked for the wisdom of the Perl Monks concerning the following question:

Yo Monks,

I'm parsing a textfile, turning the data within into a HoHoH, and using WDDX.pm (link) to output it as a WDDX (Web Distributed Data Exchange) packet--an xml document with a structure that corresponds to data types commonly used by various programming languages (including perl).

In my script, I'm using a technique that's new to me. I took it from the documentation of WDDX.pm--I assign a subroutine to a variable, and then pass the variable to a method of the wddx object. However, when I modify the subroutine, I get a very strange error from WDDX.

My code follows (the technique I mention is at line 18, which starts with my $type_sub = ...):

Sorry for the CGI and other variables that aren't being used yet--this script is destined to run as a CGI.

use strict;
use CGI;
use WDDX;
use Data::Dumper;

my $q = new CGI;
my $wddx = new WDDX;

# get params
my $report = $q->param('report');

# send request to josh's data service
my $hashref = getData($report);

#print Dumper($hashref);
#print "\n\n";

my $type_sub = sub {
    my( $name, $row, $mode ) = @_;
    #print "$name--$row--$mode\n";
    #$name =~ /wind/     and return "number";
};

my $wddx_obj = $wddx->hash2wddx( $hashref, $type_sub );

$WDDX::INDENT = "   ";
print $wddx->serialize( $wddx_obj );

exit;

sub getData {
    # here's where we'll get various query params for ea report
    my $report = shift; # dummy value

    # return a hash w data structure
    my $data;

    open (FH,"/home/httpd/html/newdev2/wddx/ib_basics.txt") or die("Co
+uldn't open: $!");
    while (<FH>) {
        if ($_ =~ /^"\@QUERY:(.*)"/) {
            my $query = $1;
            my $i = 0;
            my @keys = ();
            while (<FH>) {
                last if /^"\@ENDQUERY"/;
                $_ =~ s/\r\n//;
                $_ =~ s/\"//g;

                # get the keys
                push(@keys,split(",",$_)) unless $i ne 0;
                # reset @values and get 'em
                my @values = ();
                push(@values,split(",",$_)) unless $i eq 0;

                # fill da hashes
                for(my $k = 0; $k <= $#keys; $k++) {
                    $data->{$query}->{"r$i"}->{$keys[$k]} = $values[$k
+] unless $values[$k] eq "";
                }
                # increment the row
                $i++
            }
        }
    }

    close FH;
    return $data;
}
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The error is:

Unable to create object of type WDDX::3: Can't locate object method "n
+ew" via package "WDDX::3" (perhaps you forgot to load "WDDX::3"?) at 
+(eval 4) line 1. at text2wddx.cgi line 42
[download]

However, there is no package WDDX::3... and I thought it was a strange coincidence that in the $type_sub subroutine I split $_ into 3 variables, so I uncommented this line in my code and everything started running again:

#$name =~ /wind/     and return "number";
[download]

My question: What is this technique that I'm stealing, and why does that line confuse WDDX.pm?

For completeness, here is what I think is the eval that is bombing in WDDX.pm:

### from sub hash2wddx

my $var = eval "WDDX::\u$type->new( \$val )" or
    croak "Unable to create object of type WDDX::\u$type: " .
        _shift_blame( $@ );
$new_hash->{$name} = $var;
next;

## sub continues
[download]

Thanks,

----------

Using perl 5.6.1 unless otherwise noted.
Apache 1.3.27 unless otherwise noted.
Redhat 7.1 unless otherwise noted.
[download]

Comment on WDDX.pm weird eval error Select or Download Code

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Re: WDDX.pm weird eval error by halley (Prior) on Apr 15, 2004 at 15:08 UTC
If you do `WDDX::$type->new(...)` and it says "no such WDDX::3", then your $type has something you weren't expecting. You get your $type variable by calling a subroutine. What does the subroutine `return`? It returns whatever you tell it to return, or the last expression evaluated. Your subroutine's last line is (or was) a complicated thing which might explicitly return something, and might not. And if you comment out everything else in the sub, you're going to return the list `( $name, $row, $mode )`. In scalar context, an array is noted for its length (`3`), while a list is noted for its last element, and it's perl's guess here as to which you might want to implicitly return. -- `[ e d @ h a l l e y . c c ]`	[reply] [d/l] [select]

Replies are listed 'Best First'.

Re: WDDX.pm weird eval error
by halley (Prior) on Apr 15, 2004 at 15:08 UTC

WDDX::$type->new(...)

$type has something you weren't expecting

You get your $type variable by calling a subroutine. What does the subroutine return? It returns whatever you tell it to return, or the last expression evaluated.

Your subroutine's last line is (or was) a complicated thing which might explicitly return something, and might not. And if you comment out everything else in the sub, you're going to return the list ( $name, $row, $mode ). In scalar context, an array is noted for its length (3), while a list is noted for its last element, and it's perl's guess here as to which you might want to implicitly return.

--
[ e d @ h a l l e y . c c ]

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