Hi, Have searched around for an answer to this without success - could be I'm searching for the wrong thing but anyway... I'm trying to delay evaluation of a tied scalar to the point where it is actually used in some expression, and not just passed between subroutines. When evaluated, it's actually making a call to a database, and I'm trying to delay this call to the very last moment (e.g. when it's used in a print statement). Easiest to explain in code I guess, which looks something like this...

package lazydbvalue;

use strict;
use Carp;

sub TIESCALAR {
    # $sth is a prepared DBI statement handle
    my ($class, $sth) = @_;
    my $self = {};
    $self->{sth} = $sth;
    bless ($self, $class);
    return $self;
}

sub FETCH {
    my $self = shift;
    $self->{sth}->execute();
    my $row = $self->{sth}->fetchrow_arrayref;
    $self->{sth}->finish();
    return $row->[0];
}

sub STORE {
    my ($self, $sth) = @_;
    croak('STORE not supported - read only');
}

sub DESTROY {
    my $self = shift;
    undef $self->{sth};
}

package main;
use strict;
use DBI;

# Returns the "lazy" tied scalar
sub getNumUsers {
    my $dbh = shift;
    my $sth = $dbh->prepare('SELECT COUNT(*) FROM users');
    tie my $numUsers, 'lazydbvalue', $sth;
    return $numUsers;
}

# Connect to DB etc., assiging connection to $dbh;

my $numUsers = getNumUsers($dbh);

print "Number of users: $numUsers\n";
[download]

The problem is on returning the value from getNumUsers(), the tied FETCH method is already being called. What I want is to delay this until, say, the later print statement, when the value is actually being used, rather than just being passed. I guess the solution is somehow detecting, within the FETCH, whether it's just being passed or whether the value is actually being used. Something like;

sub FETCH {
    my $self = shift;

    # Just passing...
    if ( justPassingTheValue() ) {
        return $self;
    }

    # Value is being used in some expression
    # Now evaluate the real thing...
    $self->{sth}->execute();
    my $row = $self->{sth}->fetchrow_arrayref;
    $self->{sth}->finish();
    return $row->[0];
}
[download]

Something similar wantarray I guess - knowing the calling context. Many thanks.

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