This came up for some work being done at $JOB, which then went through a complete re-design, making this code no longer necessary. But I thought it was an interesting variation on an iterator.

It probably isn't novel to have this style of iterator wrap multiple sources and present them as a single stream. What makes this different is that the sources are read from on a rotating basis; in this case it was n DBI statement handles which represented data that was split among n different MySQL hosts due to the sheer size of the dataset. For certain (mostly aesthetic) reasons, they wanted the processing stage to interleave the result-sets rather than process them in sequence.

This is based on the wrapped objects being DBI statement handles, and the desired return-format being the sort of hash-ref structure returned by the fetchrow_hashref() method. You can write the next() as you see fit for your encapsulated objects.

For more on clever Perl-ish iterators, see chapter 4 ("Iterators") of Higher-Order Perl by Mark-Jason Dominus (ISBN 9781558607019).

# Usage: my $iter = DbiSthIterator->new($sth1, $sth2, ...);
#
#        while ($row = $iter->next()) {
#            ...undef signals the exhaustion of the iterator
#        }

package DbiSthIterator;

sub new {
    my ($class, @sth) = @_;
    bless \@sth, $class;
}

sub next {
    my $self = shift;

    while (my $sth = shift(@$self)) {
        if (my $row = $sth->fetchrow_hashref()) {
            push(@$self, $sth);
            return $row;
        } else {
            # Leaving the exhausted $sth out of the queue
            next;
        }
    }

    undef;
}

1;

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In reply to Variation on the "Higher Order Perl" Iterator Pattern by rjray

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