moderated, a problem using Tree::DAG_Node?

I was programming with the Tree::DAG_Node perl module and I was able to
get it to work up to a point where it just dropped the ball or maybe I
am blind to what I am doing wrong.

I am doing a walkdown using callback down a DAG tree. When I hit a
certain node, I wanted to process the data further. The problem is
that when I insert a foreach statement, the walkdown stops traversing
the tree.

Testing the DAG Tree output:

print map "$_\n" , @{$root->draw_ascii_tree};
produces the following diagram so I know the Tree creation was
successful:

|
<Shipment and Tracking Data>
|
<2000886>
|
<INV1000>
/--------------\
| |
<TrackUPS1234> <TrackUPS4321>

using the following code:

my $found_customer = 0;
$root->walk_down({
callback => sub {
my $node = shift;
print " " x $_[0]->{_depth};
if ( $_[0]->{_depth} eq 0 ) {
print "\n" ;
}
elsif ( $_[0]->{_depth} eq 1 ) {
if ( $found_customer eq 1 ) {
print "write footer email\n";
print " " x $_[0]->{_depth};
}
print "header email - Customer\# " . $node->name . " "
. $node-> attributes->{'name'}. " " . $node->attributes->{'email'} .
"\n" ;
$found_customer = 1;
$f0 = "";
$f1 = "";
$f2 = "";
$f3 = "";
$f4 = "";
@customized_letter_lines = "";
$f0 = $node->attributes->{'email'};
$f1 = $node->attributes->{'name'};
$f2 = $node->name;

my @letter_lines = @header_lines;

}
elsif ( $_[0]->{_depth} eq 2 ) {
print "Invoice " . $node->name . "\n";
$f3 = $node->name;
}
elsif ( $_[0]->{_depth} eq 3 ) {
print " Track " . $node->name . "\n";
$f4 = $node->name;
}
else {
1
}

},
_depth => 0
});

if ( $found_customer eq 1 ) {
print " write footer email\n";
}

output is:

header email - Customer# 2000886 Happy, Inc. XXXX@XXXXX.COM
Invoice INV1000
Track TrackUPS1234
Track TrackUPS4321
write footer email

so far so good, but when I add the foreach statement:

...
my @letter_lines = @header_lines;
#insert the foreach statement
foreach my $letter_line (@letter_lines) {
}
#end insert the foreach statement


The output is now:

header email - Customer# 2000886 Happy Inc. XXXX@XXXXX.COM
write footer email

As you can see, it did not traverse the Invoice(INV) and Track nodes.

Can you shed some light on why it stopped traversing the rest of the
nodes after the foreach statement?

Much thanks,
Jared Evans

Update:

The problem was solved: (much thanks to Roel van der Steen)

Here is what he said regarding my problem with my Tree::DAG_Node code
not fully traversing the tree:

-------------------------------------

It's not a bug, it's a feature. :)


"
This function [the callback function] must
return true or false -- if false, it will
block the next step: [...]
"

Your problem is that the foreach loop is the last statement executed in
the callback function, and the foreach returns undef, which evaluates
to false, so the walk_down procedure stops traversing. Adding a "return
1" corrects this problem. In fact, when using callback, make sure you
always explicitly set its return value with a "return" statement.