pod people versus elementalists

A long time ago, I wanted to write something to let my pod (documentation) contain its own coverage hints. I gave up when I found out that it was not going to be trivial to say something like this:

my @blocks = PodParser->read_file($my_perl_module)->data_for('coverage');

In order to extract “foo\nbar” from:

sub foo { ... }

=begin coverage


=end coverage

I found ways, but they all bugged me. I gave up on the project for a long time, because it was a real yak, but eventually I came back to it when I realized how much pod manipulation I’d want in Dist::Zilla. I really wasn’t happy with how Pod::Simple worked. Dieter had contributed a bit to Pod::Simple, and had talked about writing a more TreeBuilder-like interface. There were a number of significant blockers, though, and I didn’t want to get hung up on them. Instead, while walking to McGrady’s for ABE.pm, I had an idea and called Dieter to brainstorm with him. Basically, the idea can now be summarized as “I should write Pod::Eventual.”

Pod is really great. It’s so easy to write that I know I write much, much more documentation that I would if I had to produce, say, a chm file. It’s a very, very simple format, and is complex enough to handle almost everything I’ve ever needed from it. My problems have been that I want to write even less and have it rewritten for me, so I can avoid boilerplate.

The root problem is that pod has both very simple and very complex parts. Here are some of the simple things:

  • a pod document is made up of paragraphs
  • paragraphs are separated by blank lines (but ‘cut’ commands are special)
  • pod can be interwoven with non-pod in a document
  • pod paragraphs are either:
    • commands (start with =)
    • verbatim (start with whitespace)
    • text (start with anything else)
  • the non-whitespace characters after the = in a command are the command

So, knowing this is about enough to write a tolerable pod paragraph parser for most uses. Sure, it misses a lot of encoding stuff, but adding that later (I believe) is not a big issue.

It omits two very, very big things. First of all, it ignores the content of text paragraphs. That means that I’ve said nothing about what F<markup> means. This is a big obnoxious problem, and I have absolutely zero interest in tackling it. Hooray for punting, right?

The second problem is that it assumes that all pod documents are sequences of paragraphs. In fact, this is true. The problem is that on top of the syntax of paragraphs, there are paragraph semantics that make this, for example, an illegal document:


=item * Isn't this simple?


We have an =item outside of an =over and an =end outside of a =begin. Wait… outside? If a pod document is just a sequence of paragraphs, how does containment work?

Well, it doesn’t.

It is fairly obvious that the begin and over commands set up containment. They have start commands and end commands, and anything between the two is contained “inside” the block. Unfortunately, there’s a lot of implicit containment in how many pod formatters relate the document to the reader. For example, look at how the Sub::Exporter Cookbook is presented. head2 items are presented, in table of contents, as being contained by the head1 items. You’d also like to think that the text and verbatim paragraphs that occur between two head1 paragraphs are contained by the first. Unfortunately, that isn’t how it works, and it isn’t really clear how it should work. What items cause the end of a container? What items can contain themselves?

Again, I originally punted. Pod::Eventual just produces the sequence of events. For the things I wanted to do, however, I needed structure. I wanted to be able to make a head1 thing and put head2 or other things inside of it. (Actually, in Pod::Weaver, the technical term for these is “thingers”.)

Dieter had long since abandoned his work on pod stuff, so I stole (with his blessing) the name for my pod event-to-tree transformer: Pod::Elemental. It reads in a document that contains pod and returns a sequence of roots of trees that represent the document’s pod. The logic by which they’re formed into a structure is contained in the Nester, and anyone can write his own nester to use whatever nesting logic he thinks makes the most sense.

Pod::Weaver uses Pod::Elemental to turn a Perl document (using PPI) into a just-Perl document and a collection just-Pod elements. The elements are then reorganized and rewritten, in part by looking at the Perl and in part by using plugins and provided input. Dist::Zilla uses Pod::Weaver to add a name-and-abstract section, a license section, to build methods and attributes sections, and to do other stuff like that. It works very well, assuming you don’t mind minor explosions while I rejigger the API every other day.

Right now, I know that I have ignored a lot of what is demanded by perlpodspec. Frankly, I intend to keep ignoring a bunch of it. My goal is to let people work with pod paragraph syntax without worrying about the syntax of paragraph content or of the semantics of paragraph ordering – until they want to. The default Pod::Elemental::Nester, for example, will barf if you try to give it an =end outside matching =begin. Pod::Eventual, however, doesn’t care.

Pod::Elemental doesn’t care about other things, though, like the magic attached to =begin (data) blocks whose identifiers begin with a colon. Why? It’s just about slinging around paragraphs, not around understanding meaning.

I’m definitely planning on adding quite a bit more standards-compliance to Elemental. For one thing, I want to get =encoding hashed out and improve the interface for the element tree. Even Eventual needs some help. For example, I think it gets the definition of a blank line (which divides paragraphs) wrong, and I’d like to change how it understands the lines between =cut and a blank.

Still, though, I’m very happy with what I have and how simply I got it. I definitely would not recomment writing a pod-to-text converter using any of this code, but for writing a pod preprocessor, I’ve found it really great.

Written on October 25, 2008
🐪 perl
🏷 pod
🧑🏽‍💻 programming