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Perlweekly Confidential: Corralling News

created: July 26, 2014

This one is straight up from the Chronicles of a Lazy Man. As you might know, I'm a co-editor of the PerlWeekly. Part of the job is to curate the articles, and write blurbs about it. Another part is the aggregating of those articles in the JSON document that will be used to generate the email and webpage. As you might guess, the former is all fun and games, while the latter is... not so much.

As documented in a previous blog entry, I already eased the pain of the clerk work by using a Markdown-ish format, and having Perl figure out the title and date of publication of the articles from the url. Which is already quite nice. But I began to think... Those articles, they come mostly from blogs, right? And blogs, they mostly do have RSS feeds, right? And those feeds, they mostly do have all the metadata (title, dates, author) I'm looking for, right? So... why am I doing all that work?

Yes, dear friends, I wasn't being lazy enough. And that wouldn't do.

Gathering articles from RSS feeds

So what I did was to create a simple rss feed file, with one feed per line. Something like

# http://techblog.babyl.ca/feed/atom

With that as my source of information, I then rolled up my sleeves and began to write my script.

First, I needed to read the lines, skipping over the commented out ones.

my @feeds = map { s/\s.*$//r; } grep { /^/ and not /^\s*$/ } <>;
chomp @feeds;

Then, because the PerlWeekly always reports what happened the week before, I preemptively found out what was the cut-off date for this edition.

use DateTime::Functions qw/ today /;

my $cutout_date = today();

# find out last Monday
$cutout_date->subtract( days => 1) until $cutout_date->day_of_week == 1;

With that, I'm ready to read and filter all those feeds.

use XML::Feed;
use URI;

my %seen;

my @entries = 
    sort { $a->issued <=> $b->issued        }
    grep { not $seen{ $_->link }++          } # skip dupes
    grep { $_->issued >= $cutout_date       } # recent enough?
    map  { $_->entries                      } # get all its entries
    map  { XML::Feed->parse( URI->new($_) ) } # get the feed

The brunt of the work here is done by XML::Feed, which doesn't only fetch and parse the blog entries for us, but also transparently deal with both RSS and atom formats.

Now that we have the nicely deduped articles, all that remains to do is to write them out in as stub entries in the Markdown-like format I use.

for my $entry ( @entries ) {
    say '### ', $entry->title;
    say $entry->link;
    say eval { $entry->issued->ymd } || '????-??-??';
    say "\n", $entry->author, "\n";

And there we go. Less than 30 lines of code to take care of most of the data gathering drudgery.

Gathering RSS feeds from articles

Most sane persons would have stopped there. Me, I began to think... So now I have to gather the urls of the RSS feeds I want to harvest, right? But we have lots of sites we feature over and over again, right? So we already have urls pointing to those sites, don't we? So if we could revisit those sites, and figure out if they have a rss url...

And that's where XML::Feed's wonderful find_feeds() enter the picture. So, again, I open my editor and begin to write an itsy-bitsy script...

First, I get the location of the feed file and the PerlWeekly document I want to use.

use Path::Tiny qw/ path /;

my( $feeds_file, $week_file ) = map { path($_) } @ARGV;

From the PerlWeekly's file, I extract all articles' urls.

use JSON::Path;

my $json = $week_file->slurp;

my $jpath = JSON::Path->new( '$.chapters[*].entries[*].url' );

my @urls = map { URI->new($_) } $jpath->values($json);

I establish a list of sites we already know about from the already-recorded rss feeds.

use URI;

my %already_seen;

for ( $feeds_file->lines ) {
    next if /^\s*$/;
    $already_seen{ URI->new($_)->host }++;

Then I let XML::Feed try to discover the new feeds, and append them to the feed file.

use XML::Feed;

for my $url ( @urls ) {
    next if $already_seen{ $url->host }++;

    for my $feed ( XML::Feed->find_feeds($url) ) {
        warn "adding $feed\n";
        $feeds_file->append( "$feed\n" );


And, again, we are done. Our database of monitored sites can now grow and be harvested with a minimum of effort, and I can now focus on the important stuff. Namely: write snarky blurbs for all those articles.

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About the author

Yanick Champoux
Perl necrohacker, ACP writer, orchid lover. Slightly bonker all around. Works at Infinity Interactive