http://qs321.pair.com?node_id=1144370


in reply to reference to an undefined key

Hi exilepanda,

You can't do it the way you're trying, because during the creation of the object "$obj" your code is referring to "$obj" itself which doesn't exist yet! That's kind of like trying to define a word by using the same word in its definition: 

"A recursive algorithm is an algorithm which recursively calls itself 
+to produce an answer"

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You have a couple of choices. The first is to use the second snippet of code you showed (which you already know works, since it waits until the object $obj is fully defined before trying to reference its data).

Or you could construct the object without referencing the object itself: 


use strict;
use warnings;

package MakeObject;
use Data::Dumper;

my $rootDir = '/home';
my $userId  = 'smith';
my $appName = 'project';

my $obj = MakeObject->new;

printf "Object contents => %s\n", Dumper($obj);

sub new {
    my $obj = bless {
        Root        =>  $rootDir, 
        UserDir     => "$rootDir/Usr/$userId/",
        UserAppData => "$rootDir/Usr/$userId/$appName/",
    }, shift;

    return $obj;
}

__END__
Output is:

Object contents => $VAR1 = bless( {
                 'Root' => '/home',
                 'UserDir' => '/home/Usr/smith/',
                 'UserAppData' => '/home/Usr/smith/project/'
               }, 'MakeObject' );

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Since the latter has the same effect -- it only refers to data which is already defined ($rootDir, $userId and $appName, but not $obj) -- it doesn't create the catch 22 effect of the original code.

say  substr+lc crypt(qw $i3 SI$),4,5

Replies are listed 'Best First'.
Re^2: reference to an undefined key
by exilepanda (Friar) on Oct 10, 2015 at 17:39 UTC
    Thanks golux! your second opt is what I hope not to use as it can come really lengthy and will be my disaster if something is going to change ( eg. /Usr/ needed to change as /Users/ )... But what you propose remind me something I did yrs ago, do you have any feedback to my update on the post ?
[reply]
      You're asking if there's any module for dealing with resolving templates. I'm sure there are, but none offhand that I've used extensively. (Did you try a search of cpan?)

      As to the inherent dangers of elaborating things as you suggest, the one that comes to mind is -- you'd better only make use of key/value pairs in the order they're resolved, or you may come up against infinite recursion. For example: 

      
[ConfigDirs]
A = /some/dir
B = /_#C#_/DeeperDir
C = /_#B#_/EvenDeeperDir

      [download]

      which would never fully expand (B references C, which references B again). So you wouldn't want to necessarily make that into a hash; a list would be more appropriate I think, as you could throw an error any time you came across a variable which hadn't previously been defined.

      Here's an example that uses an array (hardwired, but you could read it in from a config file), and then creates a hash of resolved parameters: 

      #!/usr/bin/perl

###############
## Libraries ##
###############
use strict;
use warnings;
use feature qw{ say };


##################
## User-defined ##
##################
# Define what makes a "template" variable
my $re_template = qr/_#([^#]+)#_/;

# The result of what you read from your config file, perhaps.
# Note that it's an ARRAY, since we'd like to prevent unintentional
# recursion (we should resolve template variables in the order in
# which they're seen).
my $a_params = [
    [ 'A' => '/some/dir' ],
    [ 'B' => '/_#A#_/DeeperDir' ],
    [ 'C' => '/_#B#_/EvenDeeperDir' ],
];


##################
## Main program ##
##################
show_params($a_params, "[Before resolution]");
my $h_resolved = resolve_templates($a_params);
show_params($h_resolved, "[After resolution]");



#################
## Subroutines ##
#################
sub resolve_templates {
    my ($a_params) = @_;

    my $h_resolved = { };

    for (my $i = 0; $i < @$a_params; $i++) {
        my $idx          = $i + 1;
        my $a_keyval     = $a_params->[$i];
        my ($key, $tmpl) = @$a_keyval;
        my $orig         = $tmpl;

        while ($tmpl =~ /$re_template/) {
            my $ref    = $1;
            my $refval = $h_resolved->{$ref};
            defined($refval) or die "Undefined ref '$ref' in '$orig'\n
+";
            $tmpl =~ s/$re_template/$refval/g;
        }

        $h_resolved->{$key} = $tmpl;
    }

    return $h_resolved;
}


sub show_params {
    my ($a_params, $msg) = @_;

    $msg ||= "Parameters:";

    say "-" x 79;
    say $msg;
    say "-" x 79;

    if (ref $a_params eq 'HASH') {
        # If a HASH was passed, turn it into an ARRAY
        my $h_params = $a_params;
        my @keys     = sort { $a cmp $b } keys %$h_params;
        $a_params    = [ map { [ $_ => $h_params->{$_} ] } @keys ];
    }

    foreach my $a_keyval (@$a_params) {
        my ($key, $val) = @$a_keyval;
        my $text = "Parameter '$key' ";
        $text   .= "." x (32 - length($text));
        $text   .= " '$val'";
        say $text;
    }

    say "";
}

      [download]

      And here its output: 

      ----------------------------------------------------------------------
+---------
[Before resolution]
----------------------------------------------------------------------
+---------
Parameter 'A' .................. '/some/dir'
Parameter 'B' .................. '/_#A#_/DeeperDir'
Parameter 'C' .................. '/_#B#_/EvenDeeperDir'

----------------------------------------------------------------------
+---------
[After resolution]
----------------------------------------------------------------------
+---------
Parameter 'A' .................. '/some/dir'
Parameter 'B' .................. '//some/dir/DeeperDir'
Parameter 'C' .................. '///some/dir/DeeperDir/EvenDeeperDir'

      [download]

      say  substr+lc crypt(qw $i3 SI$),4,5
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