perlpod(1)
NAME
perlpod - the Plain Old Documentation format
DESCRIPTION
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
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
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
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
=pod
=head1 Heading Text
=head2 Heading Text
=head3 Heading Text
=head4 Heading Text
=over indentlevel
=item stuff
=back
=begin format
=end format
=for format text...
=encoding type
=cut
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. 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 in between 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 in between. (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,
then put a "=encoding encodingname" command very 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. Some pod formatters may try to guess
between a Latin-1 or CP-1252 versus UTF-8 encoding, but they may
guess wrong. It's best to be explicit if you use anything besides
strict ASCII. Examples:
=encoding latin1
=encoding utf8
=encoding koi8-r
=encoding ShiftJIS
=encoding big5
"=encoding" affects the whole document, and must occur only once.
And don't forget, all commands but "=encoding" last 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.
(And some older Pod translators may require the "=encoding" line to
have a following blank line as well, even though it should be legal to
omit.)
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
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($^T)>"") or some
other form of computerese (""C<drwxr-xr-x>"").
"L<name>" -- a hyperlink
There are various syntaxes, listed below. In the syntaxes given,
"text", "name", and "section" cannot contain the characters '/' and
'|'; and any '<' or '>' should be matched.
* "L<name>"
Link to a Perl manual page (e.g., "L<Net::Ping>"). Note that
"name" should not contain spaces. This syntax is also
occasionally used for references to Unix man pages, as in
"L<crontab(5)>".
* "L<name/"sec">" or "L<name/sec>"
Link to a section in other manual page. E.g., "L<perlsyn/"For
Loops">"
* "L</"sec">" or "L</sec>"
Link to a section in this manual page. E.g., "L</"Object
Methods">"
A section is started by the named heading or item. For example,
"L<perlvar/$.>" 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<postfix "if"|perlsyn/"Statement Modifiers">"
* "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:...>"
"L<text|scheme:...>"
Links to an absolute URL. For example,
"L<http://www.perl.org/>" or "L<The Perl Home
Page|http://www.perl.org/>".
"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/CP-1252 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.
Doubled angle brackets ("<<" and ">>") may be used if and only if there
is whitespace right after the opening delimiter and whitespace 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>
The multiple-bracket form does not affect the interpretation of the
contents of the formatting code, only how it must end. That means that
the examples above are also 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
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
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. The
perl executable will ignore the Pod text. You can place a Pod
statement where perl expects the beginning of a new statement, but not
within a statement, as that would result in an error. 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
*
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.
* 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.
* 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 {
...
* 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.
* 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.
* Going past the 70th column in a verbatim block might be
ungracefully wrapped by some formatters.
SEE ALSO
perlpodspec, "PODs: Embedded Documentation" in perlsyn, perlnewmod, perldoc, pod2html, pod2man, podchecker.
AUTHOR
Larry Wall, Sean M. Burke
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