PERLPOD(1) Perl Programmers Reference Guide PERLPOD(1)NAMEperlpod - 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 for-
mats 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 justi-
fied.
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 code-
block or other text which does not require any special pars-
ing 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 itali-
cize 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
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=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. (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<$/>
Such commands are explained in the "Formatting Codes"
section, below.
"=over indentlevel"
"=item stuff..."
"=back"
Item, over, and back require a little more explanation:
"=over" starts a region specifically for the generation
of a list using "=item" commands, or for indenting
(groups of) normal paragraphs. At the end of your list,
use "=back" to end it. The indentlevel option to
"=over" indicates how far over to indent, generally in
ems (where one em is the width of an "M" in the
document's base font) or roughly comparable units; if
there is no indentlevel option, it defaults to four.
(And some formatters may just ignore whatever
indentlevel you provide.) In the stuff in "=item
stuff...", you may use formatting codes, as seen here:
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=item Using C<$|> 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 con-
sistent: 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 noth-
ing 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 begin-
ning 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 any-
thing, 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
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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 tar-
getname", 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" com-
mand.
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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 for-
matting 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 "=encod-
ing 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
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=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
In ordinary paragraphs and in some command paragraphs, vari-
ous 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
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Renders code in a typewriter font, or gives some other
indication that this represents program text
(""C<gmtime($^T)>"") or some other form of computerese
(""C<drwxr-xr-x>"").
"L<name>" -- a hyperlink
There are various syntaxes, listed below. In the syn-
taxes given, "text", "name", and "section" cannot con-
tain the characters '/' and '|'; and any '<' or '>'
should be matched.
* "L<name>"
Link to a Perl manual page (e.g., "L<Net::Ping>").
Note that "name" should not contain spaces. This
syntax is also occasionally used for references to
UNIX man pages, as in "L<crontab(5)>".
* "L<name/"sec">" or "L<name/sec>"
Link to a section in other manual page. E.g.,
"L<perlsyn/"For Loops">"
* "L</"sec">" or "L</sec>" or "L<"sec">"
Link to a section in this manual page. E.g.,
"L</"Object Methods">"
A section is started by the named heading or item. For
example, "L<perlvar/$.>" 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">"
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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 format-
ting 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 inter-
preted as being in decimal, as in "E<181>".
Note that older Pod formatters might not recognize
octal or hex numeric escapes, and that many for-
matters 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>""
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"S<text>" -- text contains non-breaking spaces
This means that the words in text should not be broken
across lines. Example: "S<$x ? $y : $z>".
"X<topic name>" -- an index entry
This is ignored by most formatters, but some may use it
for building indexes. It always renders as
empty-string. Example: "X<absolutizing relative URLs>"
"Z<>" -- a null (zero-effect) formatting code
This is rarely used. It's one way to get around using
an E<...> code sometimes. For example, instead of
""NE<lt>3"" (for "N<3") you could write ""NZ<><3"" (the
"Z<>" breaks up the "N" and the "<" so they can't be
considered the part of a (fictitious) "N<...>" code.
Most of the time, you will need only a single set of angle
brackets to delimit the beginning and end of formatting
codes. However, sometimes you will want to put a real right
angle bracket (a greater-than sign, '>') inside of a format-
ting code. This is particularly common when using a format-
ting code to provide a different font-type for a snippet of
code. As with all things in Perl, there is more than one
way to do it. One way is to simply escape the closing
bracket using an "E" code:
C<$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 stan-
dard 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 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 whi-
tespace immediately follows the last '<' of the opening del-
imiter, and immediately precedes the first '>' of the clos-
ing delimiter. (The whitespace is ignored.) So the follow-
ing will also work:
C<<< $a <=> $b >>>
C<<<< $a <=> $b >>>>
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And they all mean exactly the same as this:
C<$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
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 transla-
tor 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. Perl will ignore the Pod text.
See any of the supplied library modules for examples. If
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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 transla-
tors wouldn't have recognized the "=head1" as starting a Pod
block.
Hints for Writing Pod
+ The podchecker command is provided for checking Pod syn-
tax 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 docu-
mentation in simple HTML, and converting it to Pod with
the experimental Pod::HTML2Pod module, (available in
CPAN), and looking at the resulting code. The experi-
mental 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:
# - - - - - - - - - - - -
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=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 docu-
ment 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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