perlpod - the Plain Old Documentation format
Pod is a simple-to-use markup language used for writing
documentation for Perl, Perl programs, and Perl modules.
Translators are available for converting Pod to various
formats like plain text, HTML, man pages, and more.
Pod markup consists of three basic kinds of paragraphs:
ordinary, verbatim, and command.
Ordinary Paragraph [Toc] [Back]
Most paragraphs in your documentation will be ordinary
blocks of text, like this one. You can simply type in
your text without any markup whatsoever, and with just a
blank line before and after. When it gets formatted, it
will undergo minimal formatting, like being rewrapped,
probably put into a proportionally spaced font, and maybe
even justified.
You can use formatting codes in ordinary paragraphs, for
bold, italic, "code-style", hyperlinks, and more. Such
codes are explained in the "Formatting Codes" section,
below.
Verbatim Paragraph [Toc] [Back]
Verbatim paragraphs are usually used for presenting a
codeblock or other text which does not require any special
parsing or formatting, and which shouldn't be wrapped.
A verbatim paragraph is distinguished by having its first
character be a space or a tab. (And commonly, all its
lines begin with spaces and/or tabs.) It should be reproduced
exactly, with tabs assumed to be on 8-column boundaries.
There are no special formatting codes, so you
can't italicize or anything like that. A means and
nothing else.
Command Paragraph [Toc] [Back]
A command paragraph is used for special treatment of whole
chunks of text, usually as headings or parts of lists.
All command paragraphs (which are typically only one line
long) start with "=", followed by an identifier, followed
by arbitrary text that the command can use however it
pleases. Currently recognized commands are
=head1 Heading Text
=head2 Heading Text
=head3 Heading Text
=head4 Heading Text
=over indentlevel
=item stuff
=back
=cut
=pod
=begin format
=end format
=for format text...
To explain them each in detail:
"=head1 Heading Text"
"=head2 Heading Text"
"=head3 Heading Text"
"=head4 Heading Text"
Head1 through head4 produce headings, head1 being the
highest level. The text in the rest of this paragraph
is the content of the heading. For example:
=head2 Object Attributes
The text "Object Attributes" comprises the heading
there. (Note that head3 and head4 are recent additions,
not supported in older Pod translators.) The
text in these heading commands can use formatting
codes, as seen here:
=head2 Possible Values for C<$/>
Such commands are explained in the "Formatting Codes"
section, below.
"=over indentlevel"
"=item stuff..."
"=back"
Item, over, and back require a little more explanation:
"=over" starts a region specifically for the
generation of a list using "=item" commands, or for
indenting (groups of) normal paragraphs. At the end
of your list, use "=back" to end it. The indentlevel
option to "=over" indicates how far over to indent,
generally in ems (where one em is the width of an "M"
in the document's base font) or roughly comparable
units; if there is no indentlevel option, it defaults
to four. (And some formatters may just ignore whatever
indentlevel you provide.) In the stuff in "=item
stuff...", you may use formatting codes, as seen here:
=item Using C<$|> to Control Buffering
Such commands are explained in the "Formatting Codes"
section, below.
Note also that there are some basic rules to using
"=over" ... "=back" regions:
* Don't use "=item"s outside of an "=over" ...
"=back" region.
* The first thing after the "=over" command should
be an "=item", unless there aren't going to be any
items at all in this "=over" ... "=back" region.
* Don't put "=headn" commands inside an "=over" ...
"=back" region.
* 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. -- namely, things that
look nothing like bullets or numbers.
If you start with bullets or numbers, stick with
them, as formatters use the first "=item" type to
decide how to format the list.
"=cut"
To end a Pod block, use a blank line, then a line
beginning with "=cut", and a blank line after it.
This lets Perl (and the Pod formatter) know that this
is where Perl code is resuming. (The blank line
before the "=cut" is not technically necessary, but
many older Pod processors require it.)
"=pod"
The "=pod" command by itself doesn't do much of anything,
but it signals to Perl (and Pod formatters)
that a Pod block starts here. A Pod block starts with
any command paragraph, so a "=pod" command is usually
used just when you want to start a Pod block with an
ordinary paragraph or a verbatim paragraph. For example:
=item stuff()
This function does stuff.
=cut
sub stuff {
...
}
=pod
Remember to check its return value, as in:
stuff() || die "Couldn't do stuff!";
=cut
"=begin formatname"
"=end formatname"
"=for formatname text..."
For, begin, and end will let you have regions of
text/code/data that are not generally interpreted as
normal Pod text, but are passed directly to particular
formatters, or are otherwise special. A formatter
that can use that format will use the region, otherwise
it will be completely ignored.
A command "=begin formatname", some paragraphs, and a
command "=end formatname", mean that the text/data
inbetween is meant for formatters that understand the
special format called formatname. For example,
=begin html
<hr> <img src="thang.png">
<p> This is a raw HTML paragraph </p>
=end html
The command "=for formatname text..." specifies that
the remainder of just this paragraph (starting right
after formatname) is in that special format.
=for html <hr> <img src="thang.png">
<p> This is a raw HTML paragraph </p>
This means the same thing as the above "=begin html"
... "=end html" region.
That is, with "=for", you can have only one paragraph's
worth of text (i.e., the text in "=foo targetname
text..."), but with "=begin targetname" ... "=end
targetname", you can have any amount of stuff inbetween.
(Note that there still must be a blank line
after the "=begin" command and a blank line before the
"=end" command.
Here are some examples of how to use these:
=begin html
<br>Figure 1.<br><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.)
A format name of "comment" is common for just making
notes (presumably to yourself) that won't appear in
any formatted version of the Pod document:
=for comment
Make sure that all the available options are documented!
Some formatnames will require a leading colon (as in
"=for :formatname", or "=begin :formatname" ... "=end
:formatname"), to signal that the text is not raw
data, but instead is Pod text (i.e., possibly containing
formatting codes) that's just not for normal formatting
(e.g., may not be a normal-use paragraph, but
might be for formatting as a footnote).
"=encoding encodingname"
This command is used for declaring the encoding of a
document. Most users won't need this; but if your
encoding isn't US-ASCII or Latin-1, then put a
"=encoding encodingname" command early in the document
so that pod formatters will know how to decode the
document. For encodingname, use a name recognized by
the Encode::Supported module. Examples:
=encoding utf8
=encoding koi8-r
=encoding ShiftJIS
=encoding big5
And don't forget, when using any command, that the command
lasts up until the end of its paragraph, not its line. So
in the examples below, you can see that every command
needs the blank line after it, to end its paragraph.
Some examples of lists include:
=over
=item *
First item
=item *
Second item
=back
=over
=item Foo()
Description of Foo function
=item Bar()
Description of Bar function
=back
Formatting Codes [Toc] [Back]
In ordinary paragraphs and in some command paragraphs,
various formatting codes (a.k.a. "interior sequences") can
be used:
"I<text>" -- italic text
Used for emphasis (""be I<careful!>"") and parameters
(""redo I<LABEL>"")
"B<text>" -- bold text
Used for switches (""perl's B<-n> switch""), programs
(""some systems provide a B<chfn> for that""), emphasis
(""be B<careful!>""), and so on (""and that feature
is known as B<autovivification>"").
"C<code>" -- code text
Renders code in a typewriter font, or gives some other
indication that this represents program text
(""C<gmtime($^T)>"") or some other form of computerese
(""C<drwxr-xr-x>"").
"L<name>" -- a hyperlink
There are various syntaxes, listed below. In the syntaxes
given, "text", "name", and "section" cannot contain
the characters '/' and '|'; and any '<' or '>'
should be matched.
* "L<name>"
Link to a Perl manual page (e.g., "L<Net::Ping>").
Note that "name" should not contain spaces. This
syntax is also occasionally used for references to
UNIX man pages, as in "L<crontab(5)>".
* "L<name/"sec">" or "L<name/sec>"
Link to a section in other manual page. E.g.,
"L<perlsyn/"For Loops">"
* "L</"sec">" or "L</sec>" or "L<"sec">"
Link to a section in this manual page. E.g.,
"L</"Object Methods">"
A section is started by the named heading or item.
For example, "L<perlvar/$.>" or "L<perlvar/"$.">" both
link to the section started by ""=item $."" in perlvar.
And "L<perlsyn/For Loops>" or "L<perlsyn/"For
Loops">" both link to the section started by ""=head2
For Loops"" in perlsyn.
To control what text is used for display, you use
""L<text|...>"", as in:
* "L<text|name>"
Link this text to that manual page. E.g., "L<Perl
Error Messages|perldiag>"
* "L<text|name/"sec">" or "L<text|name/sec>"
Link this text to that section in that manual
page. E.g., "L<SWITCH statements|perlsyn/"Basic
BLOCKs and Switch Statements">"
* "L<text|/"sec">" or "L<text|/sec>" or
"L<text|"sec">"
Link this text to that section in this manual
page. E.g., "L<the various attributes|/"Member
Data">"
Or you can link to a web page:
* "L<scheme:...>"
Links to an absolute URL. For example,
"L<http://www.perl.org/>". But note that there is
no corresponding "L<text|scheme:...>" syntax, for
various reasons.
"E<escape>" -- a character escape
Very similar to HTML/XML "&foo;" "entity references":
* "E<lt>" -- a literal < (less than)
* "E<gt>" -- a literal > (greater than)
* "E<verbar>" -- a literal | (vertical bar)
* "E<sol>" = a literal / (solidus)
The above four are optional except in other formatting
codes, notably "L<...>", and when preceded
by a capital letter.
* "E<htmlname>"
Some non-numeric HTML entity name, such as
"E<eacute>", meaning the same thing as "é"
in HTML -- i.e., a lowercase e with an acute
(/-shaped) accent.
* "E<number>"
The ASCII/Latin-1/Unicode character with that number.
A leading "0x" means that number is hex, as
in "E<0x201E>". A leading "0" means that number
is octal, as in "E<075>". Otherwise number is
interpreted as being in decimal, as in "E<181>".
Note that older Pod formatters might not recognize
octal or hex numeric escapes, and that many formatters
cannot reliably render characters above
255. (Some formatters may even have to use compromised
renderings of Latin-1 characters, like
rendering "E<eacute>" as just a plain "e".)
"F<filename>" -- used for filenames
Typically displayed in italics. Example:
""F<.cshrc>""
"S<text>" -- text contains non-breaking spaces
This means that the words in text should not be broken
across lines. Example: "S<$x ? $y : $z>".
"X<topic name>" -- an index entry
This is ignored by most formatters, but some may use
it for building indexes. It always renders as
empty-string. Example: "X<absolutizing relative
URLs>"
"Z<>" -- a null (zero-effect) formatting code
This is rarely used. It's one way to get around using
an E<...> code sometimes. For example, instead of
""NE<lt>3"" (for "N<3") you could write ""NZ<><3""
(the "Z<>" breaks up the "N" and the "<" so they can't
be considered the part of a (fictitious) "N<...>"
code.
Most of the time, you will need only a single set of angle
brackets to delimit the beginning and end of formatting
codes. However, sometimes you will want to put a real
right angle bracket (a greater-than sign, '>') inside of a
formatting code. This is particularly common when using a
formatting code to provide a different font-type for a
snippet of code. As with all things in Perl, there is
more than one way to do it. One way is to simply escape
the closing bracket using an "E" code:
C<$a E<lt>=E<gt> $b>
This will produce: ""$a <=> $b""
A more readable, and perhaps more "plain" way is to use an
alternate set of delimiters that doesn't require a single
">" to be escaped. With the Pod formatters that are standard
starting with perl5.5.660, doubled angle brackets
("<<" and ">>") may be used if and only if there is
whitespace right after the opening delimiter and whites-
pace right before 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. (The whitespace is ignored.) So the
following will also work:
C<<< $a <=> $b >>>
C<<<< $a <=> $b >>>>
And they all mean exactly the same as this:
C<$a E<lt>=E<gt> $b>
As a further example, this means that if you wanted to put
these bits of code in "C" (code) style:
open(X, ">>thing.dat") || die $!
$foo->bar();
you could do it like so:
C<<< open(X, ">>thing.dat") || die $! >>>
C<< $foo->bar(); >>
which is presumably easier to read than the old way:
C<open(X, "E<gt>E<gt>thing.dat") || die $!>
C<$foo-E<gt>bar();>
This is currently supported by pod2text (Pod::Text),
pod2man (Pod::Man), and any other pod2xxx or Pod::Xxxx
translators that use Pod::Parser 1.093 or later, or
Pod::Tree 1.02 or later.
The Intent [Toc] [Back]
The intent is simplicity of use, not power of expression.
Paragraphs 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 vi, or Esc Q in my version of emacs). I wanted
the translator to always leave the "'" and "`" and """
quotes alone, in verbatim mode, so I could slurp in a
working program, shift it over four spaces, and have it
print out, er, verbatim. And presumably in a monospace
font.
The Pod format is not necessarily sufficient for writing a
book. Pod is just meant to be an idiot-proof common
source for nroff, HTML, TeX, and other markup languages,
as used for online documentation. Translators exist for
pod2text, pod2html, pod2man (that's for nroff(1) and
troff(1)), pod2latex, and pod2fm. Various others are
available in CPAN.
Embedding Pods in Perl Modules [Toc] [Back]
You can embed Pod documentation in your Perl modules and
scripts. Start your documentation with an empty line, a
"=head1" command at the beginning, and end it with a
"=cut" command and an empty line. Perl will ignore the
Pod text. See any of the supplied library modules for
examples. If you're going to put your Pod 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 command.
__END__
=head1 NAME
Time::Local - efficiently compute time from local and
GMT time
Without that empty line before the "=head1", many translators
wouldn't have recognized the "=head1" as starting a
Pod block.
Hints for Writing Pod [Toc] [Back]
o The podchecker command is provided for checking Pod
syntax for errors and warnings. For example, it
checks for completely blank lines in Pod blocks and
for unknown commands and formatting codes. You should
still also pass your document 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.
o If you're more familiar with writing in HTML than with
writing in Pod, you can try your hand at writing documentation
in simple HTML, and converting it to Pod
with the experimental Pod::HTML2Pod module, (available
in CPAN), and looking at the resulting code. The
experimental Pod::PXML module in CPAN might also be
useful.
o Many older Pod translators require the lines before
every Pod command and after every Pod command (including
"=cut"!) to be a blank line. Having something
like this:
# - - - - - - - - - - - -
=item $firecracker->boom()
This noisily detonates the firecracker object.
=cut
sub boom {
...
...will make such Pod translators completely fail to
see the Pod block at all.
Instead, have it like this:
# - - - - - - - - - - - -
=item $firecracker->boom()
This noisily detonates the firecracker object.
=cut
sub boom {
...
o Some older Pod translators require paragraphs (including
command paragraphs like "=head2 Functions") to be
separated by completely empty lines. If you have an
apparently empty line with some spaces on it, this
might not count as a separator for those translators,
and that could cause odd formatting.
o Older translators might add wording around an L<>
link, so that "L<Foo::Bar>" may become "the Foo::Bar
manpage", for example. So you shouldn't write things
like "the L<foo> documentation", if you want the
translated document to read sensibly -- instead write
"the L<Foo::Bar|Foo::Bar> documentation" or "L<the
Foo::Bar documentation|Foo::Bar>", to control how the
link comes out.
o Going past the 70th column in a verbatim block might
be ungracefully wrapped by some formatters.
perlpodspec, "PODs: Embedded Documentation" in perlsyn,
perlnewmod, perldoc, pod2html, pod2man, podchecker.
Larry Wall, Sean M. Burke
perl v5.8.5 2002-11-06 12 [ Back ] |
