Pod::Simple::SubclassiUser)Contributed Perl DocumenPod::Simple::Subclassing(3)NAMEPod::Simple::Subclassing-- write a formatter as a Pod::Simple subclass
SYNOPSIS
package Pod::SomeFormatter;
use Pod::Simple;
@ISA = qw(Pod::Simple);
$VERSION = '1.01';
use strict;
sub _handle_element_start {
my($parser, $element_name, $attr_hash_r) = @_;
...
}
sub _handle_element_end {
my($parser, $element_name) = @_;
...
}
sub _handle_text {
my($parser, $text) = @_;
...
}
1;
DESCRIPTION
This document is about using Pod::Simple to write a Pod processor, gen-
erally a Pod formatter. If you just want to know about using an exist-
ing Pod formatter, instead see its documentation and see also the docs
in Pod::Simple.
The zeroeth step in writing a Pod formatter is to make sure that there
isn't already a decent one in CPAN. See <http://search.cpan.org/>, and
run a search on the name of the format you want to render to. Also con-
sider joining the Pod People list
<http://lists.perl.org/showlist.cgi?name=pod-people> and asking whether
anyone has a formatter for that format -- maybe someone cobbled one
together but just hasn't released it.
The first step in writing a Pod processor is to read perlpodspec, which
contains notes information on writing a Pod parser (which has been
largely taken care of by Pod::Simple), but also a lot of requirements
and recommendations for writing a formatter.
The second step is to actually learn the format you're planning to for-
mat to -- or at least as much as you need to know to represent Pod,
which probably isn't much.
The third step is to pick which of Pod::Simple's interfaces you want to
use -- the basic interface via Pod::Simple or Pod::Simple::Methody is
event-based, sort of like HTML::Parser's interface, or sort of like
XML::Parser's "Handlers" interface), but Pod::Simple::PullParser pro-
vides a token-stream interface, sort of like HTML::TokeParser's inter-
face; Pod::Simple::SimpleTree provides a simple tree interface, rather
like XML::Parser's "Tree" interface. Users familiar with XML-handling
will find one of these styles relatively familiar; but if you would be
even more at home with XML, there are classes that produce an XML rep-
resentation of the Pod stream, notably Pod::Simple::XMLOutStream; you
can feed the output of such a class to whatever XML parsing system you
are most at home with.
The last step is to write your code based on how the events (or tokens,
or tree-nodes, or the XML, or however you're parsing) will map to con-
structs in the output format. Also sure to consider how to escape text
nodes containing arbitrary text, and also what to do with text nodes
that represent preformatted text (from verbatim sections).
Events
TODO intro... mention that events are supplied for implicits, like for
missing >'s
In the following section, we use XML to represent the event structure
associated with a particular construct. That is, TODO
"$parser->_handle_element_start( element_name, attr_hashref )"
"$parser->_handle_element_end( element_name )"
"$parser->_handle_text( text_string )"
TODO describe
events with an element_name of Document
Parsing a document produces this event structure:
<Document start_line="543">
...all events...
</Document>
The value of the start_line attribute will be the line number of
the first Pod directive in the document.
If there is no Pod in the given document, then the event structure
will be this:
<Document contentless="1" start_line="543">
</Document>
In that case, the value of the start_line attribute will not be
meaningful; under current implementations, it will probably be the
line number of the last line in the file.
events with an element_name of Para
Parsing a plain (non-verbatim, non-directive, non-data) paragraph
in a Pod document produces this event structure:
<Para start_line="543">
...all events in this paragraph...
</Para>
The value of the start_line attribute will be the line number of
the start of the paragraph.
For example, parsing this paragraph of Pod:
The value of the I<start_line> attribute will be the
line number of the start of the paragraph.
produces this event structure:
<Para start_line="129">
The value of the
<I>
start_line
</I>
attribute will be the line number of the first Pod directive
in the document.
</Para>
events with an element_name of B, C, F, or I.
Parsing a B<...> formatting code (or of course any of its semanti-
cally identical syntactic variants B<< ... >>, or B<<<< ... >>>>,
etc.) produces this event structure:
<B>
...stuff...
</B>
Currently, there are no attributes conveyed.
Parsing C, F, or I codes produce the same structure, with only a
different element name.
If your parser object has been set to accept other formatting
codes, then they will be presented like these B/C/F/I codes --
i.e., without any attributes.
events with an element_name of S
Normally, parsing an S<...> sequence produces this event structure,
just as if it were a B/C/F/I code:
<S>
...stuff...
</S>
However, Pod::Simple (and presumably all derived parsers) offers
the "nbsp_for_S" option which, if enabled, will suppress all S
events, and instead change all spaces in the content to non-break-
ing spaces. This is intended for formatters that output to a format
that has no code that means the same as S<...>, but which has a
code/character that means non-breaking space.
events with an element_name of X
Normally, parsing an X<...> sequence produces this event structure,
just as if it were a B/C/F/I code:
<X>
...stuff...
</X>
However, Pod::Simple (and presumably all derived parsers) offers
the "nix_X_codes" option which, if enabled, will suppress all X
events and ignore their content. For formatters/processors that
don't use X events, this is presumably quite useful.
events with an element_name of L
Because the L<...> is the most complex construct in the language,
it should not surprise you that the events it generates are the
most complex in the language. Most of complexity is hidden away in
the attribute values, so for those of you writing a Pod formatter
that produces a non-hypertextual format, you can just ignore the
attributes and treat an L event structure like a formatting element
that (presumably) doesn't actually produce a change in formatting.
That is, the content of the L event structure (as opposed to its
attributes) is always what text should be displayed.
There are, at first glance, three kinds of L links: URL, man, and
pod.
When a L<some_url> code is parsed, it produces this event struc-
ture:
<L content-implicit="yes" to="that_url" type="url">
that_url
</L>
The "type="url"" attribute is always specified for this type of L
code.
For example, this Pod source:
L<http://www.perl.com/CPAN/authors/>
produces this event structure:
<L content-implicit="yes" to="http://www.perl.com/CPAN/authors/" type="url">
http://www.perl.com/CPAN/authors/
</L>
When a L<manpage(section)> code is parsed (and these are fairly
rare and not terribly useful), it produces this event structure:
<L content-implicit="yes" to="manpage(section)" type="man">
manpage(section)
</L>
The "type="man"" attribute is always specified for this type of L
code.
For example, this Pod source:
L<crontab(5)>
produces this event structure:
<L content-implicit="yes" to="crontab(5)" type="man">
crontab(5)
</L>
In the rare cases where a man page link has a specified, that text
appears in a section attribute. For example, this Pod source:
L<crontab(5)/"ENVIRONMENT">
will produce this event structure:
<L content-implicit="yes" section="ENVIRONMENT" to="crontab(5)" type="man">
"ENVIRONMENT" in crontab(5)
</L>
In the rare case where the Pod document has code like L<some-
text|manpage(section)>, then the sometext will appear as the con-
tent of the element, the manpage(section) text will appear only as
the value of the to attribute, and there will be no "con-
tent-implicit="yes"" attribute (whose presence means that the Pod
parser had to infer what text should appear as the link text -- as
opposed to cases where that attribute is absent, which means that
the Pod parser did not have to infer the link text, because that L
code explicitly specified some link text.)
For example, this Pod source:
L<hell itself!|crontab(5)>
will produce this event structure:
<L to="crontab(5)" type="man">
hell itself!
</L>
The last type of L structure is for links to/within Pod documents.
It is the most complex because it can have a to attribute, or a
section attribute, or both. The "type="pod"" attribute is always
specified for this type of L code.
In the most common case, the simple case of a L<podpage> code pro-
duces this event structure:
<L content-implicit="yes" to="Net::Ping" type="pod">
podpage
</L>
For example, this Pod source:
L<Net::Ping>
produces this event structure:
<L content-implicit="yes" to="Net::Ping" type="pod">
Net::Ping
</L>
In cases where there is link-text explicitly specified, it is to be
found in the content of the element (and not the attributes), just
as with the L<sometext|manpage(section)> case discussed above. For
example, this Pod source:
L<Perl Error Messages|perldiag>
produces this event structure:
<L to="perldiag" type="pod">
Perl Error Messages
</L>
In cases of links to a section in the current Pod document, there
is a section attribute instead of a to attribute. For example,
this Pod source:
L</"Member Data">
produces this event structure:
<L content-implicit="yes" section="Member Data" type="pod">
"Member Data"
</L>
As another example, this Pod source:
L<the various attributes|/"Member Data">
produces this event structure:
<L section="Member Data" type="pod">
the various attributes
</L>
In cases of links to a section in a different Pod document, there
are both a section attribute and a to attribute. For example, this
Pod source:
L<perlsyn/"Basic BLOCKs and Switch Statements">
produces this event structure:
<L content-implicit="yes" section="Basic BLOCKs and Switch Statements" to="perlsyn" type="pod">
"Basic BLOCKs and Switch Statements" in perlsyn
</L>
As another example, this Pod source:
L<SWITCH statements|perlsyn/"Basic BLOCKs and Switch Statements">
produces this event structure:
<L section="Basic BLOCKs and Switch Statements" to="perlsyn" type="pod">
SWITCH statements
</L>
Incidentally, note that we do not distinguish between these syn-
taxes:
L</"Member Data">
L<"Member Data">
L</Member Data>
L<Member Data> [deprecated syntax]
That is, they all produce the same event structure, namely:
<L content-implicit="yes" section="Member Data" type="pod">
"Member Data"
</L>
events with an element_name of E or Z
While there are Pod codes E<...> and Z<>, these do not produce any
E or Z events -- that is, there are no such events as E or Z.
events with an element_name of Verbatim
When a Pod verbatim paragraph (AKA "codeblock") is parsed, it pro-
duces this event structure:
<Verbatim start_line="543" xml:space="preserve">
...text...
</Verbatim>
The value of the start_line attribute will be the line number of
the first line of this verbatim block. The xml:space attribute is
always present, and always has the value "preserve".
The text content will have tabs already expanded.
events with an element_name of head1 .. head4
When a "=head1 ..." directive is parsed, it produces this event
structure:
<head1>
...stuff...
</head1>
For example, a directive consisting of this:
=head1 Options to C<new> et al.
will produce this event structure:
<head1 start_line="543">
Options to
<C>
new
</C>
et al.
</head1>
"=head2" thru "=head4" directives are the same, except for the ele-
ment names in the event structure.
events with an element_name of over-bullet
When an "=over ... =back" block is parsed where the items are a
bulletted list, it will produce this event structure:
<over-bullet indent="4" start_line="543">
<item-bullet start_line="545">
...Stuff...
</item-bullet>
...more item-bullets...
</over-bullet>
The value of the indent attribute is whatever value is after the
"=over" directive, as in "=over 8". If no such value is specified
in the directive, then the indent attribute has the value "4".
For example, this Pod source:
=over
=item *
Stuff
=item *
Bar I<baz>!
=back
produces this event structure:
<over-bullet indent="4" start_line="10">
<item-bullet start_line="12">
Stuff
</item-bullet>
<item-bullet start_line="14">
Bar <I>baz</I>!
</item-bullet>
</over-bullet>
events with an element_name of over-number
When an "=over ... =back" block is parsed where the items are a
numbered list, it will produce this event structure:
<over-number indent="4" start_line="543">
<item-number number="1" start_line="545">
...Stuff...
</item-number>
...more item-number...
</over-bullet>
This is like the "over-bullet" event structure; but note that the
contents are "item-number" instead of "item-bullet", and note that
they will have a "number" attribute, which some formatters/proces-
sors may ignore (since, for example, there's no need for it in HTML
when producing an "<UL><LI>...</LI>...</UL>" structure), but which
any processor may use.
Note that the values for the number attributes of "item-number"
elements in a given "over-number" area will start at 1 and go up by
one each time. If the Pod source doesn't follow that order (even
though it really should should!), whatever numbers it has will be
ignored (with the correct values being put in the number
attributes), and an error message might be issued to the user.
events with an element_name of over-text
These events are are somewhat unlike the other over-* structures,
as far as what their contents are. When an "=over ... =back" block
is parsed where the items are a list of text "subheadings", it will
produce this event structure:
<over-text indent="4" start_line="543">
<item-text>
...stuff...
</item-text>
...stuff (generally Para or Verbatim elements)...
<item-text>
...more item-text and/or stuff...
</over-text>
The indent attribute is as with the other over-* events.
For example, this Pod source:
=over
=item Foo
Stuff
=item Bar I<baz>!
Quux
=back
produces this event structure:
<over-text indent="4" start_line="20">
<item-text start_line="22">
Foo
</item-text>
<Para start_line="24">
Stuff
</Para>
<item-text start_line="26">
Bar
<I>
baz
</I>
!
</item-text>
<Para start_line="28">
Quux
</Para>
</over-text>
events with an element_name of over-block
These events are are somewhat unlike the other over-* structures,
as far as what their contents are. When an "=over ... =back" block
is parsed where there are no items, it will produce this event
structure:
<over-block indent="4" start_line="543">
...stuff (generally Para or Verbatim elements)...
</over-block>
The indent attribute is as with the other over-* events.
For example, this Pod source:
=over
For cutting off our trade with all parts of the world
For transporting us beyond seas to be tried for pretended offenses
He is at this time transporting large armies of foreign mercenaries to
complete the works of death, desolation and tyranny, already begun with
circumstances of cruelty and perfidy scarcely paralleled in the most
barbarous ages, and totally unworthy the head of a civilized nation.
=cut
will produce this event structure:
<over-block indent="4" start_line="2">
<Para start_line="4">
For cutting off our trade with all parts of the world
</Para>
<Para start_line="6">
For transporting us beyond seas to be tried for pretended offenses
</Para>
<Para start_line="8">
He is at this time transporting large armies of [...more text...]
</Para>
</over-block>
events with an element_name of item-bullet
See "events with an element_name of over-bullet", above.
events with an element_name of item-number
See "events with an element_name of over-number", above.
events with an element_name of item-text
See "events with an element_name of over-text", above.
events with an element_name of for
TODO...
events with an element_name of Data
TODO...
More Pod::Simple Methods
Pod::Simple provides a lot of methods that aren't generally interesting
to the end user of an existing Pod formatter, but some of which you
might find useful in writing a Pod formatter. They are listed below.
The first several methods (the accept_* methods) are for declaring the
capabilites of your parser, notably what "=for targetname" sections
it's interested in, what extra N<...> codes it accepts beyond the ones
described in the perlpod.
"$parser->accept_targets( SOMEVALUE )"
As the parser sees sections like:
=for html <img src="fig1.jpg">
or
=begin html
<img src="fig1.jpg">
=end html
...the parser will ignore these sections unless your subclass has
specified that it wants to see sections targetted to "html" (or
whatever the formatter name is).
If you want to process all sections, even if they're not targetted
for you, call this before you start parsing:
$parser->accept_targets('*');
"$parser->accept_targets_as_text( SOMEVALUE )"
This is like accept_targets, except that it specifies also that the
content of sections for this target should be treated as Pod text
even if the target name in "=for targetname" doesn't start with a
":".
At time of writing, I don't think you'll need to use this.
"$parser->accept_codes( Codename, Codename... )"
This tells the parser that you accept additional formatting codes,
beyond just the standard ones (I B C L F S X, plus the two weird
ones you don't actually see in the parse tree, Z and E). For exam-
ple, to also accept codes "N", "R", and "W":
$parser->accept_codes( qw( N R W ) );
TODO: document how this interacts with =extend, and long element
names
"$parser->accept_directive_as_data( directive_name )"
"$parser->accept_directive_as_verbatim( directive_name )"
"$parser->accept_directive_as_processed( directive_name )"
In the unlikely situation that you need to tell the parser that you
will accept additional directives ("=foo" things), you need to
first set the parset to treat its content as data (i.e., not really
processed at all), or as verbatim (mostly just expanding tabs), or
as processed text (parsing formatting codes like B<...>).
For example, to accept a new directive "=method", you'd presumably
use:
$parser->accept_directive_as_processed("method");
so that you could have Pod lines like:
=method I<$whatever> thing B<um>
Making up your own directives breaks compatibility with other Pod
formatters, in a way that using "=for target ..." lines doesn't;
however, you may find this useful if you're making a Pod superset
format where you don't need to worry about compatibility.
"$parser->nbsp_for_S( BOOLEAN );"
Setting this attribute to a true value (and by default it is false)
will turn "S<...>" sequences into sequences of words separated by
"\xA0" (non-breaking space) characters. For example, it will take
this:
I like S<Dutch apple pie>, don't you?
and treat it as if it were:
I like DutchE<nbsp>appleE<nbsp>pie, don't you?
This is handy for output formats that don't have anything quite
like an "S<...>" code, but which do have a code for non-breaking
space.
There is currently no method for going the other way; but I can
probably provide one upon request.
"$parser->version_report()"
This returns a string reporting the $VERSION value from your module
(and its classname) as well as the $VERSION value of Pod::Simple.
Note that perlpodspec requires output formats (wherever possible)
to note this detail in a comment in the output format. For exam-
ple, for some kind of SGML output format:
print OUT "<!-- \n", $parser->version_report, "\n -->";
"$parser->pod_para_count()"
This returns the count of Pod paragraphs seen so far.
"$parser->line_count()"
This is the current line number being parsed. But you might find
the "line_number" event attribute more accurate, when it is
present.
"$parser->nix_X_codes( SOMEVALUE )"
This attribute, when set to a true value (and it is false by
default) ignores any "X<...>" sequences in the document being
parsed. Many formats don't actually use the content of these
codes, so have no reason to process them.
"$parser->merge_text( SOMEVALUE )"
This attribute, when set to a true value (and it is false by
default) makes sure that only one event (or token, or node) will be
created for any single contiguous sequence of text. For example,
consider this somewhat contrived example:
I just LOVE Z<>hotE<32>apple pie!
When that is parsed and events are about to be called on it, it may
actually seem to be four different text events, one right after
another: one event for "I just LOVE ", one for "hot", one for " ",
and one for "apple pie!". But if you have merge_text on, then
you're guaranteed that it will be fired as one text event: "I just
LOVE hot apple pie!".
"$parser->code_handler( CODE_REF )"
This specifies code that should be called when a code line is seen
(i.e., a line outside of the Pod). Normally this is undef, meaning
that no code should be called. If you provide a routine, it should
start out like this:
sub get_code_line { # or whatever you'll call it
my($line, $line_number, $parser) = @_;
...
}
Note, however, that sometimes the Pod events aren't processed in
exactly the same order as the code lines are -- i.e., if you have a
file with Pod, then code, then more Pod, sometimes the code will be
processed (via whatever you have code_handler call) before the all
of the preceding Pod has been processed.
"$parser->cut_handler( CODE_REF )"
This is just like the code_handler attribute, except that it's for
"=cut" lines, not code lines. The same caveats apply. "=cut" lines
are unlikely to be interesting, but this is included for complete-
ness.
"$parser->whine( linenumber, complaint string )"
This notes a problem in the Pod, which will be reported to in the
"Pod Errors" section of the document and/or send to STDERR, depend-
ing on the values of the attributes "no_whining", "no_errata_sec-
tion", and "complain_stderr".
"$parser->scream( linenumber, complaint string )"
This notes an error like "whine" does, except that it is not sup-
pressable with "no_whining". This should be used only for very
serious errors.
"$parser->source_dead(1)"
This aborts parsing of the current document, by switching on the
flag that indicates that EOF has been seen. In particularly dras-
tic cases, you might want to do this. It's rather nicer than just
calling "die"!
"$parser->hide_line_numbers( SOMEVALUE )"
Some subclasses that indescriminately dump event attributes (well,
except for ones beginning with "~") can use this object attribute
for refraining to dump the "start_line" attribute.
"$parser->no_whining( SOMEVALUE )"
This attribute, if set to true, will suppress reports of non-fatal
error messages. The default value is false, meaning that com-
plaints are reported. How they get reported depends on the values
of the attributes "no_errata_section" and "complain_stderr".
"$parser->no_errata_section( SOMEVALUE )"
This attribute, if set to true, will suppress generation of an
errata section. The default value is false -- i.e., an errata sec-
tion will be generated.
"$parser->complain_stderr( SOMEVALUE )"
This attribute, if set to true will send complaints to STDERR. The
default value is false -- i.e., complaints do not go to STDERR.
"$parser->bare_output( SOMEVALUE )"
Some formatter subclasses use this as a flag for whether output
should have prologue and epilogue code omitted. For example, set-
ting this to true for an HTML formatter class should omit the
"<html><head><title>...</title><body>..." prologue and the
"</body></html>" epilogue.
If you want to set this to true, you should probably also set
"no_whining" or at least "no_errata_section" to true.
"$parser->preserve_whitespace( SOMEVALUE )"
If you set this attribute to a true value, the parser will try to
preserve whitespace in the output. This means that such formatting
conventions as two spaces after periods will be preserved by the
parser. This is primarily useful for output formats that treat
whitespace as significant (such as text or *roff, but not HTML).
SEE ALSO
Pod::Simple -- event-based Pod-parsing framework
Pod::Simple::Methody -- like Pod::Simple, but each sort of event calls
its own method (like "start_head3")
Pod::Simple::PullParser -- a Pod-parsing framework like Pod::Simple,
but with a token-stream interface
Pod::Simple::SimpleTree -- a Pod-parsing framework like Pod::Simple,
but with a tree interface
Pod::Simple::Checker -- a simple Pod::Simple subclass that reads docu-
ments, and then makes a plaintext report of any errors found in the
document
Pod::Simple::DumpAsXML -- for dumping Pod documents as tidily indented
XML, showing each event on its own line
Pod::Simple::XMLOutStream -- dumps a Pod document as XML (without
introducing extra whitespace as Pod::Simple::DumpAsXML does).
Pod::Simple::DumpAsText -- for dumping Pod documents as tidily indented
text, showing each event on its own line
Pod::Simple::LinkSection -- class for objects representing the values
of the TODO and TODO attributes of L<...> elements
Pod::Escapes -- the module the Pod::Simple uses for evaluating E<...>
content
Pod::Simple::Text -- a simple plaintext formatter for Pod
Pod::Simple::TextContent -- like Pod::Simple::Text, but makes no effort
for indent or wrap the text being formatted
perlpod
perlpodspec
perldoc
COPYRIGHT AND DISCLAIMERS
Copyright (c) 2002 Sean M. Burke. All rights reserved.
This library is free software; you can redistribute it and/or modify it
under the same terms as Perl itself.
This program is distributed in the hope that it will be useful, but
without any warranty; without even the implied warranty of mer-
chantability or fitness for a particular purpose.
AUTHOR
Sean M. Burke "sburke@cpan.org"
perl v5.8.8 2006-07-08 Pod::Simple::Subclassing(3)
