source: trunk/third/perl/pod/perlpod.pod @ 14545

=head1 NAME

perlpod - plain old documentation

=head1 DESCRIPTION

A pod-to-whatever translator reads a pod file paragraph by paragraph,
and translates it to the appropriate output format.  There are
three kinds of paragraphs:
L<verbatim|/"Verbatim Paragraph">,
L<command|/"Command Paragraph">, and
L<ordinary text|/"Ordinary Block of Text">.

=head2 Verbatim Paragraph

A verbatim paragraph, distinguished by being indented (that is,
it starts with space or tab).  It should be reproduced exactly,
with tabs assumed to be on 8-column boundaries.  There are no
special formatting escapes, so you can't italicize or anything
like that.  A \ means \, and nothing else.

=head2 Command Paragraph

All command paragraphs start with "=", followed by an
identifier, followed by arbitrary text that the command can
use however it pleases.  Currently recognized commands are

    =head1 heading
    =head2 heading
    =item text
    =over N
    =back
    =cut
    =pod
    =for X
    =begin X
    =end X

=over 4

=item =pod

=item =cut

The "=pod" directive does nothing beyond telling the compiler to lay
off parsing code through the next "=cut".  It's useful for adding
another paragraph to the doc if you're mixing up code and pod a lot.

=item =head1

=item =head2

Head1 and head2 produce first and second level headings, with the text in
the same paragraph as the "=headn" directive forming the heading description.

=item =over

=item =back

=item =item

Item, over, and back require a little more explanation: "=over" starts a
section specifically for the generation of a list using "=item" commands. At
the end of your list, use "=back" to end it. You will probably want to give
"4" as the number to "=over", as some formatters will use this for indentation.
This should probably be a default. Note also that there are some basic rules
to using =item: don't use them outside of an =over/=back block, use at least
one inside an =over/=back block, you don't _have_ to include the =back if
the list just runs off the document, and perhaps most importantly, keep the
items consistent: either use "=item *" for all of them, to produce bullets,
or use "=item 1.", "=item 2.", etc., to produce numbered lists, or use
"=item foo", "=item bar", etc., i.e., things that looks nothing like bullets
or numbers. If you start with bullets or numbers, stick with them, as many
formatters use the first "=item" type to decide how to format the list.

=item =for

=item =begin

=item =end

For, begin, and end let you include sections that are not interpreted
as pod text, but passed directly to particular formatters. A formatter
that can utilize that format will use the section, otherwise it will be
completely ignored.  The directive "=for" specifies that the entire next
paragraph is in the format indicated by the first word after
"=for", like this:

 =for html <br>
  <p> This is a raw HTML paragraph </p>

The paired commands "=begin" and "=end" work very similarly to "=for", but
instead of only accepting a single paragraph, all text from "=begin" to a
paragraph with a matching "=end" are treated as a particular format.

Here are some examples of how to use these:

 =begin html

 <br>Figure 1.<IMG SRC="figure1.png"><br>

 =end html

 =begin text

   ---------------
   |  foo        |
   |        bar  |
   ---------------

 ^^^^ Figure 1. ^^^^

 =end text

Some format names that formatters currently are known to accept include
"roff", "man", "latex", "tex", "text", and "html". (Some formatters will
treat some of these as synonyms.)

And don't forget, when using any command, that the command lasts up until
the end of the B<paragraph>, not the line. Hence in the examples below, you
can see the empty lines after each command to end its paragraph.

Some examples of lists include:

 =over 4

 =item *

 First item

 =item *

 Second item

 =back

 =over 4

 =item Foo()

 Description of Foo function

 =item Bar()

 Description of Bar function

 =back

=back

=head2 Ordinary Block of Text

It will be filled, and maybe even
justified.  Certain interior sequences are recognized both
here and in commands:

    I<text>     Italicize text, used for emphasis or variables
    B<text>     Embolden text, used for switches and programs
    S<text>     Text contains non-breaking spaces
    C<code>     Render code in a typewriter font, or give some other
                indication that this represents program text
    L<name>     A link (cross reference) to name
                    L<name>             manual page
                    L<name/ident>       item in manual page
                    L<name/"sec">       section in other manual page
                    L<"sec">            section in this manual page
                                        (the quotes are optional)
                    L</"sec">           ditto
                same as above but only 'text' is used for output.
                (Text can not contain the characters '/' and '|', 
                and should contain matched '<' or '>')
                    L<text|name>
                    L<text|name/ident>
                    L<text|name/"sec">
                    L<text|"sec">
                    L<text|/"sec">

    F<file>     Used for filenames
    X<index>    An index entry
    Z<>         A zero-width character
    E<escape>   A named character (very similar to HTML escapes)
                    E<lt>               A literal <
                    E<gt>               A literal >
                    E<sol>              A literal /
                    E<verbar>           A literal |
                    (these are optional except in other interior
                     sequences and when preceded by a capital letter)
                    E<n>                Character number n (probably in ASCII)
                    E<html>             Some non-numeric HTML entity, such
                                        as E<Agrave>

Most of the time, you will only need a single set of angle brackets to
delimit the beginning and end of interior sequences.  However, sometimes
you will want to put a right angle bracket (or greater-than sign '>')
inside of a sequence.  This is particularly common when using a sequence
to provide a different font-type for a snippet of code.  As with all
things in Perl, there is more than one way to do it.  One way is to
simply escape the closing bracket using an C<E> sequence:

    C<$a E<lt>=E<gt> $b>

This will produce: "C<$a E<lt>=E<gt> $b>"

A more readable, and perhaps more "plain" way is to use an alternate set of
delimiters that doesn't require a ">" to be escaped.  As of perl5.5.660,
doubled angle brackets ("<<" and ">>") may be used I<if and only if there
is whitespace immediately following the opening delimiter and immediately
preceding the closing delimiter!> For example, the following will do the
trick:

    C<< $a <=> $b >>

In fact, you can use as many repeated angle-brackets as you like so
long as you have the same number of them in the opening and closing
delimiters, and make sure that whitespace immediately follows the last
'<' of the opening delimiter, and immediately precedes the first '>' of
the closing delimiter.  So the following will also work:

    C<<< $a <=> $b >>>
    C<<<< $a <=> $b >>>>

This is currently supported by pod2text (Pod::Text), pod2man (Pod::Man),
and any other pod2xxx and Pod::Xxxx translator that uses Pod::Parser
1.093 or later.


=head2 The Intent

That's it.  The intent is simplicity, not power.  I wanted paragraphs
to look like paragraphs (block format), so that they stand out
visually, and so that I could run them through fmt easily to reformat
them (that's F7 in my version of B<vi>).  I wanted the translator (and not
me) to worry about whether " or ' is a left quote or a right quote
within filled text, and I wanted it to leave the quotes alone, dammit, in
verbatim mode, so I could slurp in a working program, shift it over 4
spaces, and have it print out, er, verbatim.  And presumably in a
constant width font.

In particular, you can leave things like this verbatim in your text:

    Perl
    FILEHANDLE
    $variable
    function()
    manpage(3r)

Doubtless a few other commands or sequences will need to be added along
the way, but I've gotten along surprisingly well with just these.

Note that I'm not at all claiming this to be sufficient for producing a
book.  I'm just trying to make an idiot-proof common source for nroff,
TeX, and other markup languages, as used for online documentation.
Translators exist for B<pod2man>  (that's for nroff(1) and troff(1)),
B<pod2text>, B<pod2html>, B<pod2latex>, and B<pod2fm>.

=head2 Embedding Pods in Perl Modules

You can embed pod documentation in your Perl scripts.  Start your
documentation with a "=head1" command at the beginning, and end it
with a "=cut" command.  Perl will ignore the pod text.  See any of the
supplied library modules for examples.  If you're going to put your
pods at the end of the file, and you're using an __END__ or __DATA__
cut mark, make sure to put an empty line there before the first pod
directive.

    __END__

    =head1 NAME

    modern - I am a modern module

If you had not had that empty line there, then the translators wouldn't
have seen it.

=head2 Common Pod Pitfalls

=over 4

=item *

Pod translators usually will require paragraphs to be separated by
completely empty lines.  If you have an apparently empty line with
some spaces on it, this can cause odd formatting.

=item *

Translators will mostly add wording around a LE<lt>E<gt> link, so that
C<LE<lt>foo(1)E<gt>> becomes "the I<foo>(1) manpage", for example (see
B<pod2man> for details).  Thus, you shouldn't write things like C<the
LE<lt>fooE<gt> manpage>, if you want the translated document to read
sensibly.

If you need total control of the text used for a link in the output
use the form LE<lt>show this text|fooE<gt> instead.

=item *

The B<podchecker> command is provided to check pod syntax
for errors and warnings. For example, it checks for completely
blank lines in pod segments and for unknown escape sequences.
It is still advised to pass it through
one or more translators and proofread the result, or print out the
result and proofread that.  Some of the problems found may be bugs in
the translators, which you may or may not wish to work around.

=back

=head1 SEE ALSO

L<pod2man>, L<perlsyn/"PODs: Embedded Documentation">,
L<podchecker>

=head1 AUTHOR

Larry Wall