perlpod - Online in the Cloud

PROGRAM:

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
"&eacute;" 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.

Use perlpod online using onworks.net services