[% setvar title Perl should use XML for documentation instead of POD %]
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Perl should use XML for documentation instead of POD
Maintainer: Frank Tobin <email@example.com> Date: 30 Sep 2000 Last Modified: 3 Oct 2000 Mailing List: firstname.lastname@example.org Number: 357 Version: 2 Status: Frozen
Perl documentation should move to using XML as the formatting language, instead of using POD. XML has many advantages over POD, and would address several problems POD is reaching as it expands beyond its original designs.
A lot of good, heated discussion was generated on the mailing lists. The majority seems against using XML-DTD documentation, but granted there are deficiencies in POD. Translating between XML and POD seemed popular. Elsewise, no really new topics were covered that the RFC doesn't address, just refined and re-hashed and such, so I'll let this RFC stand as-is.
POD, described in perlpod, is currently the de-facto language for Perl-related documentation. It is a simple language, with simple tags. According to "The Intent" in perlpod, simplicity was behind the intended design of POD. There exist several tools to convert POD into HTML, manpages, text, and other languages.
However, POD can be confusing to learn, and has many limitations.
XML, on the other hand, is a document language that is designed
for high extensibility, and little ambiguity. XML is flexible,
relying on Document Type Definitions, DTD's,
(recently, also XML Schemas), which define
the document structure being used by the author.
For example, a copy of the DTD for XHTML may be found at
Knowledge of how to write or work with DTD's is not necessary for
an XML-author; the author need only know the effect of the
DTD or XML Schema.
In the following few sections I will try to compare various aspects of POD versus XML.
POD's in-line tags use the general form
Primarily, XML uses balanced tags, e.g.,
<tag>body</tag>. POD's in-line tags tend to
be single-letters, and this can be thought of an argument
for the readability of POD in it's un-rendered form, but
an XML DTD for Perl documentation could also define
single-letter tags to minimize this.
POD's tagging, althrough simple, can produce confusing code.
For example, to render the following (
I need to have (C<tagE<lt>bodyE<gt>>),
which is horrendous in its own (look at the source to this document).
It is especially hard to understand, since one would expect
bodyE to be an inseparable string; yet, reading, one needs
to use look-ahead to see what is following the E.
XML, on the other hand, uses & as the escaping mechanism, helping a reader sort-out deeply-nested escapings.
Several of POD's tags do not necessarily relate to the meaning
of the text, but how the text is rendered. For instance,
I<text> is used to italicize text, not give meaning,
although perlpod mentions that this should be used for emphasis
This sort of style in having tags say how something should be rendered is something XHTML and friends have tried to get away from, instead having the presentation of the document separate from the structuring/tagging of it. XHTML and XML user-agents use stylesheets to render documents separately from the document itself; this allows the developer to merely give meaning to the document, instead of deciding how it will be rendered; the choice of how to render is left to the user, with the use of a stylesheet.
The use of using tags solely to give meaning also helps accessibility problems for the visually-impaired; for example, having bold text (e.g., B<foobar>) is not quite as meaningful as strongly-emphasized text (e.g, <strong>Using meaning makes things accessible!</strong>).
The use of stylesheets along with XML allows documents to be rendered very well in a variety of circumstances, such as manpages, a continuous-display browser (e.g., web-browser), and in printed form.
This can be confusing for many, as whitespace in most languages does not have functionality. However, in POD, lines beginning with a whitespace character are treated as pre-formatted text. This has already caused an RFC to take effect, RFC 216.
XML, on the other hand, uses properties to determine the whitespace handling between different types of tags; for example, a tag in a Perl XML DTD tag such as <perl> could be used to surround pre-formatted Perl code.
Who knows what languages people will base their knowledge off of in 2 years? Noone really does, but HTML-style, balanced-tag languages are a good guess though, given the web's popularity, and easy-to-grasp notion of having balanced-tags. On the other hand, POD will be Yet Another Language to learn, distinct from other typing systems the user will know. While this may not seem like a problem since POD is designed to just be a simple language, will likely become more complicated in the future (see "Extensibility").
POD has the advantage in that it has syntax that is quick and easy to type, such as C<$var++> instead of an XML/XHTML <code>var++<code>.
However, author effort pays off immensely on the reader's end. By using a structured, widely-supported language such as XML, the reader is able to render the document using a rich set of tools such as Style Sheets. Just as producing quality code generally requires effort, for different reasons, producing quality, valuable documentation also requires effort.
There exist many tools to handle XML documents, and given the popularity of XML, there will likely be many useful applications which handle XML in new and innovative means in the future, furthering the usefulness of any XML documentation. On the other hand, POD appears bound to Perl, and while translators exist, the documentation is not nearly as powerful as it could be if were XML.
POD has grown over the years, supporting more and more features. This has caused strange syntaxes to develop, such as <foo/bar> differing from <foo/"bar">.
XML designed specifically for extensibility; in fact, XHTML by itself largely surpasses POD in terms of functionality. One can expect more problems to arise as more features are added to POD, likening it possibly to the beginnings of HTML, which in time has turned into XHTML, an XML DTD.
XML also supports many features such as embedded elements, which are not well-defined in POD, and may wish to become more prominent in documentation in the future.
POD has a good advantage in that it's design allows for it to embed well into code. If documentation is to be alongside code, a direction to consider is to have the Perl program be an XML document itself, with the code inside of it, between designated tags. This would allow for the entire Perl program/document to be rendered in a unified manner, using one tool, and conforming to one meta-language, XML.
POD has been fine up to now, as it is a quick-and-dirty documentation system. However, as Perl matures, its documentation system needs to also, as it is outgrowing it's original design.
XML is mature in the sense that it is the culmination of years of experience of what to do and not do with documentation systems (e.g., older versions of HTML (presentation-oriented), and SGML (too complicated)). Using XML will solve a lot of future problems, and make documentation much more valuable.
RFC 216: POD should tolerate white space.
RFC 280: Teak POD's C<>
RFC 306: User-definable POD handling