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Perl Documentation [DRAFT]

Synopsis_26 - Perl Documentation [DRAFT]


Synopsis_26 - Perl Documentation [DRAFT]


Brian Ingerson <>


 Maintainer:    Brian Ingerson <>

 Date:          9 Apr 2005

 Last Modified: 9 Apr 2005

This document attempts to describe the documentation capabilities of Perl 6. It assumes familiarity with Perl 5 and the Pod (Plain Old Documentation) format.

NOTE: This document is based heavily on the ideas and discussions of those involved in the Perldoc project, and lightly on the views of the Perl 6 design team proper. In other words, expect things to change when Larry gets more involved.


Throughout this document, the term ``Perldoc'' will be used as the generic term to describe Perl Documentation rather than the original name ``Pod''. Pod now refers to a specific dialect of Perldoc. With Perldoc, there's more than one way to do documentation (TMTOWTDD).

This document covers the following major areas:

Perldoc Object Model (PDOM)
In Perl 6 there is a Document Object Model for Perldoc. This refers to both the fact that all Perl documentation is modeled in a certain fashion (or schema), and also to the runtime API for accessing the content of documents.

Syntax Containment
How Perl syntax is distinguished from Perldoc syntax.

Syntax Dialects
Various Perldoc syntax formats that map to the same PDOM.

Escaping and Embedding
Each Perldoc dialect syntax must provide mechanisms for escaping markup as actual content, and for embedding other dialects within itself.

Changes in Pod
There are slight changes to the Perl 5 Pod structure, to make it consistent and unambiguous.

The Kwid Dialect
Kwid is a completely new syntax based on experience from more modern internet social communication.

PDOM Extensions
The PDOM can be extended to support structures that are beyond the scope of traditional Pod.


Perldoc is centered on the notion of allowing multiple documentation dialects, but insisting that they are parsed into a consistent information model. The information can then be exposed or transformed in a consistent, well known manner. This will facilitate the creation of powerful Perldoc tools.

This information model (known as the Perldoc Object Model or PDOM) is almost exactly the one that Perl 5's Pod implicitly defines.

The PDOM can be thought of as a tree of nodes. There are 4 kinds of nodes:

Text Nodes
Leaf nodes containing content text.

Collection Nodes
Nodes that contain other nodes.

Opaque Nodes
Leaf nodes that represent something that is not part of the PDOM, but may be resolved by some other process at some other time. This might include tables, images, diagrams or raw html. Opaque nodes are typically handled by PDOM extensions, described later on.

Ignorable Nodes
Nodes for text in the syntax presentation that has no bearing on the document's intended content, but must be preserved for applications like editors and syntax hilighting. This typically includes extra whitespace and throwaway comments.

There are two categories of collection nodes:

Block Nodes
These are nodes that correspond in nature to HTML DIVs. They represent things like paragraphs, verbatim blocks, lists and list items.

Phrase Nodes
These are nodes that correspond in nature to HTML SPANs. They represent things like bold, italic, inline code and links.

Each node has a type that indicates what type of data it holds. The following is a list of nodes that exist in the PDOM model:

    - heading1_block

    - heading2_block

    - heading3_block

    - heading4_block

    - paragraph_block

    - verbatim_block

    - comment_block

    - opaque_block

    - unordered_list

    - ordered_list

    - definition_list

    - list_item

    - item_term

    - item_definition

    - bold_phrase

    - italic_phrase

    - code_phrase

    - file_phrase

    - opaque_phrase

    - document_link

    - hyper_link

    - plain_text


Perldoc allows for SAX-style streaming parsing and emission of documents. The serial API looks something like this:

    - start_document(title)  - Start a new Perldoc document

    - end_document()         - End a Perdoc document

    - start_element(type)    - Start a new node

    - end_element(type)      - End a node

    - characters(text)       - Content text as unicode chars

    - ignorable(text)        - Non-content text

PDOM Random Access API

This API consists of functions that would likely look similar to XML/DOM.

Larry has also stated that the documentation of a program will be available through the global variable %*POD which I would humbly suggest be changed or (at least aliased) to %*DOC. It has not yet been determined how all the parts of the PDOM would be accessed through this hash.

Perhaps the variable $*DOC could hold a reference to the programs PDOM object.


The first thing to tackle is how various interpreters (including the Perl interpreter) distinguish which characters in a file or stream are actual Perl code and which ones are Perldoc.

Perldoc attempts to stay within the same bounds as those imposed by the Perl 5 interpreter, namely that a section of Perldoc begins with a line matching the regexp:


and ends with the next line matching:


Perldoc currently keeps this same restriction for two reasons:

Backwards compatability with Perl 5. There is no reason why Perldoc dialects and tools cannot be used with Perl 5 today, without any need to change the interpreter.

To gain acceptance with the Perl 6 design team, by not asking for anything special to accomplish its goals. That said, the containment rules could and should be made smarter by the Perl 6 team.

Pod has a generic identifier to start a Pod section:


Note that the =pod and =cut lines are not considered part of the Pod, but simply as containment markers.

However Pod also allows a section to begin with any number of block identifiers as long as it starts with an equals sign.

So the line:

    =head2 Something To Say

acts not only as a containment starting marker, but also as part of the content (a heading).

Containment Differences in Perldoc

In general Perldoc is backwards compatible with Pod. This gives the Pod dialect a slight advantage in being able to start a section with actual content. Any other dialect that wanted this feature would need to have similar block markup.

Perldoc extends the notion of containment while still fitting inside the Perl5/Pod restrictions. Perldoc offers a generic starting marker of:


This is a dialect agnostic version of the traditional:


which is still valid in Perldoc but is a shortcut for:




is long for:


The term ``doc'' is more readily understood by those readers not familiar with Pod or Kwid.

If =doc has no dialect qualifier, it is assumed to be the dialect of the previous section. If there is no previous section, the dialect should be autodetected.

All of the text following the =doc marker but on the same line is considered to be the first line of the actual content. This allows Kwid to do some thing like this:

    =doc - Something

    Some interesting point.


to be a synonym for:


    - Something

    Some interesting point.



which is semantically equivalent to Pod's

    =item Something

    Some interesting point.


File Containment

The above describes how to divine the Perl from the Doc, which assumes they are intertwingled in a Perl source code file. Documentation can also live in a file by itself.

Perldoc considers files ending with .pod to be documentation in the Pod dialect and files ending with .kwid to be in the Kwid dialect, etc. A perldoc parser can look to the file extension for a dialect hint, if no other clue is provided.

This implies that the lines like:





are not necessary in pure Perldoc files. In fact, in a Kwid file, they would just be plain text.


In the spirit of TMTOWTDI, Perldoc allows an author to chose a documentation syntax of their choice without needing to worry whether downstream processes and tools will be able to use it properly. These variations of syntax are referred to as Perldoc Dialects.

Background and Rationale

Pod was created in a time before modern day phenomenons like wikis existed. Wikis are similar to Pod in that that they ask authors to write content prose and structural/formatting markup in an all text format that is simpler and less foreboding than HTML. Then some program converts the text into a nicely readable format like HTML.

Wiki syntax comes in dozens of varieties, but the main theme is ``make the unformatted text feel as close as possible to the formatted text, because most of the people using wikis will not be technical''. Normal non-programmmer folk aren't all that good at picking out cryptic markup from content. And while the authors of most Perldoc are very technical, some of them wonder why they can't just use the friendlier markup.

Other Dialects

``Pod'' is now the Perldoc dialect that looks exactly like Pod.

``Kwid'' is one Perldoc dialect that takes the best ideas from the various wiki syntaxes that correspond to ideas in the Pod model.

Other dialects should be created by people who are neither fond of Pod nor Kwid.

An XML dialect would be trivial to define since the PDOM can be thought of as being an XML schema.

Likewise an HTML dialect would be useful as a formal syntax for creating Pod from HTML.

A WYSIWYG Perldoc editor could be thought of as just another dialect.


In addition to providing syntactical constructs for all the nodes of the PDOM, a Perldoc dialect must provide forms for escaping plain text and embedding other Perldoc dialects (as well as opaque structures).


Escaping means to mark characters which are semantically part of the content of the document but might otherwise be construed as markup.

Here are some examples where the first line is ambiguous or wrong and the following line(s) fixes it.

In the Pod dialect:

    =head1 Not a heading

    E<eq>head1 Not a heading

    This equation C<a > b>

    This equation C<a E<gt> b>

    This equation C<< a > b >>

    This is verbatim X<<< >>>X

    This is verbatim XE<lt><< >>>X

    Perldoc(tm) is fun

    PerldocE<trade> is fun

In the Kwid dialect:

    = Not a heading

    \= Not a heading

    Not a link: [title|page/section]

    Not a link: \[title|page/section]

    This is not huggy but should be *bold*/italic/

    This is not huggy but should be {*bold*}{/italic/}

    This is verbatim {*zyz*}

    This is verbatim {\*zyz*}

    This is verbatim { {*zyz*} }

    Perldoc(tm) is fun

    Perldoc&trade; is fun 


Note that none of these is purported to be elegant, but a complete

syntax requires such mechanisms.


Each dialect must have a mechanism to switch parsing to another dialect and back again.

Using Pod and Kwid again, here is an example of Pod embedding Kwid:


    =head2 Here is a list

    =begin kwid

    * one

    * two

    * three

    =end kwid

    That was a B<list>!


and here is the opposite:


    == Here is a list



    =item *


    =item *


    =item *




    That was a *list*!



In order to make POD more consistent, the following minor details will change:

Allow named hyperlinks
Pod allows this syntax:

    L<text to show|document_name>

for named links to other documents. It should also allow:

    L<text to show|>;

for named hyperlinks.

=over and =back will be deprecated.
This markers are ambiguous as indenters and list markers.

Instead we will have:

    =begin list

    =end list

with special syntax to make them less verbose.

For more information on how the Pod dialect might change, see

If the Pod dialect is changed significantly by the Perl 6 design team, it is suggested that there remain a legacy Perl 5 dialect. Hopefully the legacy dialect would be called ``Pod'', and the improved version something else. ``Mod''??


The Kwid dialect is more formally described here:

The quickest way to explain Kwid is simply to show a side by side Pod/Kwid cheat sheet:

    =head1 Big Thing                    = Big Thing

    =head4 Small Thing                  ==== Small Thing

    A paragraph of                      A paragraph of

    plain text.                         plain text.

        # verbatim                          # verbatim

        sub v {                             sub v {

            shift;                              shift;

        }                                   }

    =over                               * foo

                                        * bar

    =item *


    =item *



    =over                               - foo

    =item foo                           Foo is free

                                        - bar  Bar is he

    Foo is free

    =item bar

    Bar is he


    Something B<bold>!                  Something *bold*!

    Something I<italic>!                Something /italic/!

    Some code C<E = M * C ^ 2>!         Some code `E = M * C ^ 2`!

    =begin opaque                       .opaque

    =end opaque                         ..opaque

    =for opaque                         .:opaque

This is just a small example to give you an idea. Kwid is really nice for nested lists:

    * This

    ++ One

    --- One  Is the /lonelist/ integer

    --- Won  The Race

    ++ Two

    --- Two  For the show

    --- Too  Far from here

    * That

    * The Other

The above in Pod would be horribly long.


All Perldoc dialects and tools are required to support all of the core constructs defined in the PDOM schema. It is assumed that data in any dialect should be able to round trip semantically when converted to any other dialect and back.

It is also intended that there will be extension libraries to add syntax parsing, schema definition, and formatting/conversion capabilities for various constructs that fall outside of the core PDOM.

Tables are a prime example. While too unweildy to impose on every tool, tables are useful in many documentation applications. So there will be an extension that handle tables.

If tools are not available at a particular stage of processing an extension construct, that construct will be reported as an opaque object by the PDOM. It is entirely possible that a further stage of proceesing will be able to move the opaque object into some representation dictated by the extension's schema.

Pod and Kwid define marker syntax for block level extensions.

    =begin foo

    =end foo


Kwid also defines syntax for phrase level extensions.

    This was written: {date: 2005-04-09}

So dates can have extension processors to do fancy things. Pod should have such a generic phrase syntax:

    This was written: <DATE><2005-04-09>

or something similar.