Flavio Poletti bio photo

Flavio Poletti

Irreducible Perler.

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One of the most effective ways to debug and refine my code is to… write about it. I am so smart that I figured it out all alone… at about 43!

(I mean fourty-three, wow!, not 6.0415263e+52).

Data::Tubes, Again and Again

I’m working on a project called Data::Tubes lately. To be honest, there’s not much left to do with respect to my original plan, so I’m actually more writing about it than writing it now.

While doing this, two things are becoming more and more evident:

  • this project is not much relevant. I see the value of it, and I’ll probably use it quite extensively to solve a class of problems that pop up from time to time, but overall it’s quite niche-y;
  • to really make something useful, it’s better to use it. A lot of the changes in the interface happened while writing examples.

The first aspect is a bummer to some extent; on the flip side, I’m having fun, learning something, keeping my programming muscles exercised and… doing something that most probably will be useful at some time.

The second one is just rediscovering what I probably read over and over, with relations to Test Driven Developemnt to some extent. I’m really not sticking to the KISS principle - it turns out that many times I’m adding things “just because it’s easy” and blends well, not out of real need. On the other hand, writing about it let me refine the interface to something handy as opposed to simply doable.

Frustration!

For relevancy of the project, here’s were the hubris really gets put to test. Well, some humility never hurt anyone.

When I first saw a reference in issue 245 of Perl Weekly I was a bit disapponted to see the whole idea summarized as templating engine using tags that resemble the tags of Template::Toolkit and input data in PSKV format. That might stand for Pipe Separated Keys and Values.. Wait… what? The whole point of Data::Tubes is to give you all the flexibility on how you read, process, render and write your outputs… how come?!?

I’m not blaming Gabor of course. The Perl Weekly team does a really remarkable job of keeping people updated on what happens in the Perl world, and it was of course my fault to not provide clear information about my project.

After that, I saw that Data::Tubes was featured in this issue of Perl Tricks. Wow, they called it a cute data transformation module… how kind! But wait… what? What does needs iterators mean?!? They’re there since the beginning!!!

Again, I think that the authors at Perl Tricks do an amazing job… again, my fault for not making things clear enough!

So What?

So… what did I do? Well, I thought that the whole frustration thing could mean two things:

  • I really suck at synthesizing and describe the gist of things. I still have to figure out how to improve here…
  • all the stuff I coded needed much better documentation, not only the reference type (the POD was already in place), but the kind that you would initially turn to to understand how a thing can be actually useful to address a problem.

This resulted in a lot of writing (the web site, the articles in the wiki)… again confirming my distance from synthesys!

All this writing benefited me in two ways:

  • I wrote something that will help me to use my project effectively and efficiently when I will need it. I’m trying to write stuff in the way I’d like to read it!
  • I got better insight at what it can do and how it can be done better - more features to add, wow!

The Birth of bucket

One such thing that happened lately eventually led to the addition of the bucket type of tap. Let’s go in steps!

Data::Tubes lets you define a sequence, or pipeline, of transforming actions called tubes. Each tube receives as input exactly one record and outputs:

  • (): the input record is simply discarded;
  • ($onething): the $onething is the output record, that is suitable for being fed to the following tube;
  • (records => \@manythings): the input record gave rise to a sequence of output records, returned as an array reference;
  • (iterator => sub{...}): the input record gave rise to a promise to generate zero, one or more output records

During the design of pipeline I had to deal with the generic case, where you don’t know in anticipation what the tubes in the pipeline will return. The most sensible choice seemed to be the iterator: it allows returning whatever number of output records (so it’s general enough) and it also does not assume that you want to go all the way down in one single shot (which is what an iterator is useful for).

So, when we define a pipeline like this:

use Data::Tubes qw< pipeline >;
my $pipeline = pipeline(
   'Source::iterate_files',
   'Reader::by_line',
   ['Parser::by_format' => 'name,age'],
   [
      'Renderer::with_template_perlish',
      "Hi [% name %]! You're [% age %] today!\n",
   ],
   'Writer::to_files'
);

we get back a tube whose output contract is the iterator one:

my @outcome = $pipeline->(\@filenames);
# $outcome[0] is the string 'iterator'
# $outcome[1] is an iterator. NO real computation happened yet!

while (my ($out) = $outcome[1]->()) {
    # ... use the $out-put record if needed
}

One thing that was immediately evident, though, was that in most cases what I wanted was to just run the whole thing for all input records. This initially led me to add the drain function, which takes care to feed a tube with inputs and drain whatever comes out of it:

use Data::Tubes qw< pipeline drain >;

# create the pipeline as before, then instead of calling this:
#    my @outcome = $pipeline->(\@filenames);
# you can call this:
drain($pipeline, \@filenames);

This is a drag (you have to import drain!), so a couple of options were added to automate the draining process. The most basic is tap, that when set to sink ensures that the iterator is drained on the spot and nothing is returned (to save memory):

use Data::Tubes qw< pipeline >;
my $pipeline = pipeline(
   'Source::iterate_files',
   'Reader::by_line',
   ['Parser::by_format' => 'name,age'],
   [
      'Renderer::with_template_perlish',
      "Hi [% name %]! You're [% age %] today!\n",
   ],
   'Writer::to_files',
   {tap => 'sink'}
);
$pipeline->(\@filenames);

So far so good.

While writing an article about alternatives, though, I came to discovering that pipeline is not only useful to define the outer pipeline, i.e. the one that does the end-to-end processing, but also to define sub-pipelines to be fed as alternatives.

In a nutshell, alternatives allows you to provide a list of alternative tubes, each of which will be tried over the same input record until one of them accepts it and returns something back.

This led me to a bug though, because:

  • by default, pipeline returns a tube that always returns an iterator. Hence, the first tube in the alternatives would always get the input record, even if it would eventually toss it away at iterator firing time;
  • working around the iterator setting the tap to sink will make all the alternatives to be always fired, because the sink does not return anything.

While it was already possible to address the problem with the available interface (pipeline also supported a pump option that could be used effectively), there was clearly a need for both exhausting the iterator and getting the output records. Hence, the following example does use alternatives in the right way:

my $template_for_OK = ...;
my $template_for_NOT_OK = ...;
pipeline(
   'Source::iterate_files',
   'Reader::by_line',
   ['Parser::by_format' => 'status:name'],
   [
      'Plumbing::alternatives',
    
      # First alternative, try to see if it's a good one
      pipeline(
          sub { # filter only good ones
             return $_[0] if $_[0]{structured}{status} eq 'OK;
             return;
          },
          ['Renderer::with_template_perlish' => $template_for_OK],
          {tap => 'bucket'}, # <<< LOOK HERE!!!
      ),
    
      # as a fallback, we render for NOT OK
      ['Renderer::with_template_perlish' => $template_for_NOT_OK],
   ],
   'Writer::to_files',
   {tap => 'sink'}, # we can just toss the records away here
)->(\@input_filenames);

Conclusions?

I don’t think I have some conclusions for this article to be honest. I’m having fun with Data::Tubes, although I start doubting its relevance and usefulness. But one thing is sure: for me, it’s an amazing learning tool as a hobby programmer!