PERLPODSPEC(1) Perl Programmers Reference Guide PERLPODSPEC(1) #
PERLPODSPEC(1) Perl Programmers Reference Guide PERLPODSPEC(1)
NNAAMMEE #
perlpodspec - Plain Old Documentation: format specification and notes
DDEESSCCRRIIPPTTIIOONN #
This document is detailed notes on the Pod markup language. Most people
will only have to read perlpod to know how to write in Pod, but this
document may answer some incidental questions to do with parsing and
rendering Pod.
In this document, "must" / "must not", "should" / "should not", and "may"
have their conventional (cf. RFC 2119) meanings: "X must do Y" means that
if X doesn't do Y, it's against this specification, and should really be
fixed. "X should do Y" means that it's recommended, but X may fail to do
Y, if there's a good reason. "X may do Y" is merely a note that X can do
Y at will (although it is up to the reader to detect any connotation of
"and I think it would be _n_i_c_e if X did Y" versus "it wouldn't really
_b_o_t_h_e_r me if X did Y").
Notably, when I say "the parser should do Y", the parser may fail to do
Y, if the calling application explicitly requests that the parser _n_o_t do
Y. I often phrase this as "the parser should, by default, do Y." This
doesn't _r_e_q_u_i_r_e the parser to provide an option for turning off whatever
feature Y is (like expanding tabs in verbatim paragraphs), although it
implicates that such an option _m_a_y be provided.
PPoodd DDeeffiinniittiioonnss Pod is embedded in files, typically Perl source files, although you can write a file that’s nothing but Pod.
A lliinnee in a file consists of zero or more non-newline characters,
terminated by either a newline or the end of the file.
A nneewwlliinnee sseeqquueennccee is usually a platform-dependent concept, but Pod
parsers should understand it to mean any of CR (ASCII 13), LF (ASCII 10),
or a CRLF (ASCII 13 followed immediately by ASCII 10), in addition to any
other system-specific meaning. The first CR/CRLF/LF sequence in the file
may be used as the basis for identifying the newline sequence for parsing
the rest of the file.
A bbllaannkk lliinnee is a line consisting entirely of zero or more spaces (ASCII
32) or tabs (ASCII 9), and terminated by a newline or end-of-file. A
nnoonn--bbllaannkk lliinnee is a line containing one or more characters other than
space or tab (and terminated by a newline or end-of-file).
(_N_o_t_e_: Many older Pod parsers did not accept a line consisting of
spaces/tabs and then a newline as a blank line. The only lines they
considered blank were lines consisting of _n_o _c_h_a_r_a_c_t_e_r_s _a_t _a_l_l,
terminated by a newline.)
WWhhiitteessppaaccee is used in this document as a blanket term for spaces, tabs,
and newline sequences. (By itself, this term usually refers to literal
whitespace. That is, sequences of whitespace characters in Pod source,
as opposed to "E<32>", which is a formatting code that _d_e_n_o_t_e_s a
whitespace character.)
A PPoodd ppaarrsseerr is a module meant for parsing Pod (regardless of whether
this involves calling callbacks or building a parse tree or directly
formatting it). A PPoodd ffoorrmmaatttteerr (or PPoodd ttrraannssllaattoorr) is a module or
program that converts Pod to some other format (HTML, plaintext, TeX,
PostScript, RTF). A PPoodd pprroocceessssoorr might be a formatter or translator, or
might be a program that does something else with the Pod (like counting
words, scanning for index points, etc.).
Pod content is contained in PPoodd bblloocckkss. A Pod block starts with a line
that matches "m/\A=[a-zA-Z]/", and continues up to the next line that
matches "m/\A=cut/" or up to the end of the file if there is no
"m/\A=cut/" line.
Note that a parser is not expected to distinguish between something that
looks like pod, but is in a quoted string, such as a here document.
Within a Pod block, there are PPoodd ppaarraaggrraapphhss. A Pod paragraph consists
of non-blank lines of text, separated by one or more blank lines.
For purposes of Pod processing, there are four types of paragraphs in a
Pod block:
• A command paragraph (also called a "directive"). The first line of
this paragraph must match "m/\A=[a-zA-Z]/". Command paragraphs are
typically one line, as in:
=head1 NOTES
=item *
But they may span several (non-blank) lines:
=for comment
Hm, I wonder what it would look like if
you tried to write a BNF for Pod from this.
=head3 Dr. Strangelove, or: How I Learned to
Stop Worrying and Love the Bomb
_S_o_m_e command paragraphs allow formatting codes in their content
(i.e., after the part that matches "m/\A=[a-zA-Z]\S*\s*/"), as in:
=head1 Did You Remember to C<use strict;>?
In other words, the Pod processing handler for "head1" will apply the
same processing to "Did You Remember to C<use strict;>?" that it
would to an ordinary paragraph (i.e., formatting codes like "C<...>")
are parsed and presumably formatted appropriately, and whitespace in
the form of literal spaces and/or tabs is not significant.
• A vveerrbbaattiimm ppaarraaggrraapphh. The first line of this paragraph must be a
literal space or tab, and this paragraph must not be inside a "=begin
_i_d_e_n_t_i_f_i_e_r", ... "=end _i_d_e_n_t_i_f_i_e_r" sequence unless "_i_d_e_n_t_i_f_i_e_r"
begins with a colon (":"). That is, if a paragraph starts with a
literal space or tab, but _i_s inside a "=begin _i_d_e_n_t_i_f_i_e_r", ... "=end
_i_d_e_n_t_i_f_i_e_r" region, then it's a data paragraph, unless "_i_d_e_n_t_i_f_i_e_r"
begins with a colon.
Whitespace _i_s significant in verbatim paragraphs (although, in
processing, tabs are probably expanded).
• An oorrddiinnaarryy ppaarraaggrraapphh. A paragraph is an ordinary paragraph if its
first line matches neither "m/\A=[a-zA-Z]/" nor "m/\A[ \t]/", _a_n_d if
it's not inside a "=begin _i_d_e_n_t_i_f_i_e_r", ... "=end _i_d_e_n_t_i_f_i_e_r" sequence
unless "_i_d_e_n_t_i_f_i_e_r" begins with a colon (":").
• A ddaattaa ppaarraaggrraapphh. This is a paragraph that _i_s inside a "=begin
_i_d_e_n_t_i_f_i_e_r" ... "=end _i_d_e_n_t_i_f_i_e_r" sequence where "_i_d_e_n_t_i_f_i_e_r" does
_n_o_t begin with a literal colon (":"). In some sense, a data
paragraph is not part of Pod at all (i.e., effectively it's "out-of-
band"), since it's not subject to most kinds of Pod parsing; but it
is specified here, since Pod parsers need to be able to call an event
for it, or store it in some form in a parse tree, or at least just
parse _a_r_o_u_n_d it.
For example: consider the following paragraphs:
# <- that's the 0th column
=head1 Foo
Stuff
$foo->bar
=cut
Here, "=head1 Foo" and "=cut" are command paragraphs because the first
line of each matches "m/\A=[a-zA-Z]/". "_[_s_p_a_c_e_]_[_s_p_a_c_e_]$foo->bar" is a
verbatim paragraph, because its first line starts with a literal
whitespace character (and there's no "=begin"..."=end" region around).
The "=begin _i_d_e_n_t_i_f_i_e_r" ... "=end _i_d_e_n_t_i_f_i_e_r" commands stop paragraphs
that they surround from being parsed as ordinary or verbatim paragraphs,
if _i_d_e_n_t_i_f_i_e_r doesn't begin with a colon. This is discussed in detail in
the section "About Data Paragraphs and "=begin/=end" Regions".
PPoodd CCoommmmaannddss This section is intended to supplement and clarify the discussion in “Command Paragraph” in perlpod. These are the currently recognized Pod commands:
"=head1", "=head2", "=head3", "=head4", "=head5", "=head6"
This command indicates that the text in the remainder of the
paragraph is a heading. That text may contain formatting codes.
Examples:
=head1 Object Attributes
=head3 What B<Not> to Do!
Both "=head5" and "=head6" were added in 2020 and might not be
supported on all Pod parsers. Pod::Simple 3.41 was released on
October 2020 and supports both of these providing support for all
Pod::Simple-based Pod parsers.
"=pod"
This command indicates that this paragraph begins a Pod block. (If
we are already in the middle of a Pod block, this command has no
effect at all.) If there is any text in this command paragraph after
"=pod", it must be ignored. Examples:
=pod
This is a plain Pod paragraph.
=pod This text is ignored.
"=cut"
This command indicates that this line is the end of this previously
started Pod block. If there is any text after "=cut" on the line, it
must be ignored. Examples:
=cut
=cut The documentation ends here.
=cut
# This is the first line of program text.
sub foo { # This is the second.
It is an error to try to _s_t_a_r_t a Pod block with a "=cut" command. In
that case, the Pod processor must halt parsing of the input file, and
must by default emit a warning.
"=over"
This command indicates that this is the start of a list/indent
region. If there is any text following the "=over", it must consist
of only a nonzero positive numeral. The semantics of this numeral is
explained in the "About =over...=back Regions" section, further
below. Formatting codes are not expanded. Examples:
=over 3
=over 3.5
=over
"=item"
This command indicates that an item in a list begins here.
Formatting codes are processed. The semantics of the (optional) text
in the remainder of this paragraph are explained in the "About
=over...=back Regions" section, further below. Examples:
=item
=item *
=item *
=item 14
=item 3.
=item C<< $thing->stuff(I<dodad>) >>
=item For transporting us beyond seas to be tried for pretended
offenses
=item He is at this time transporting large armies of foreign
mercenaries to complete the works of death, desolation and
tyranny, already begun with circumstances of cruelty and perfidy
scarcely paralleled in the most barbarous ages, and totally
unworthy the head of a civilized nation.
"=back"
This command indicates that this is the end of the region begun by
the most recent "=over" command. It permits no text after the
"=back" command.
"=begin formatname"
"=begin formatname parameter"
This marks the following paragraphs (until the matching "=end
formatname") as being for some special kind of processing. Unless
"formatname" begins with a colon, the contained non-command
paragraphs are data paragraphs. But if "formatname" _d_o_e_s begin with
a colon, then non-command paragraphs are ordinary paragraphs or data
paragraphs. This is discussed in detail in the section "About Data
Paragraphs and "=begin/=end" Regions".
It is advised that formatnames match the regexp
"m/\A:?[-a-zA-Z0-9_]+\z/". Everything following whitespace after the
formatname is a parameter that may be used by the formatter when
dealing with this region. This parameter must not be repeated in the
"=end" paragraph. Implementors should anticipate future expansion in
the semantics and syntax of the first parameter to
"=begin"/"=end"/"=for".
"=end formatname"
This marks the end of the region opened by the matching "=begin
formatname" region. If "formatname" is not the formatname of the
most recent open "=begin formatname" region, then this is an error,
and must generate an error message. This is discussed in detail in
the section "About Data Paragraphs and "=begin/=end" Regions".
"=for formatname text..."
This is synonymous with:
=begin formatname
text...
=end formatname
That is, it creates a region consisting of a single paragraph; that
paragraph is to be treated as a normal paragraph if "formatname"
begins with a ":"; if "formatname" _d_o_e_s_n_'_t begin with a colon, then
"text..." will constitute a data paragraph. There is no way to use
"=for formatname text..." to express "text..." as a verbatim
paragraph.
"=encoding encodingname"
This command, which should occur early in the document (at least
before any non-US-ASCII data!), declares that this document is
encoded in the encoding _e_n_c_o_d_i_n_g_n_a_m_e, which must be an encoding name
that Encode recognizes. (Encode's list of supported encodings, in
Encode::Supported, is useful here.) If the Pod parser cannot decode
the declared encoding, it should emit a warning and may abort parsing
the document altogether.
A document having more than one "=encoding" line should be considered
an error. Pod processors may silently tolerate this if the not-first
"=encoding" lines are just duplicates of the first one (e.g., if
there's a "=encoding utf8" line, and later on another "=encoding
utf8" line). But Pod processors should complain if there are
contradictory "=encoding" lines in the same document (e.g., if there
is a "=encoding utf8" early in the document and "=encoding big5"
later). Pod processors that recognize BOMs may also complain if they
see an "=encoding" line that contradicts the BOM (e.g., if a document
with a UTF-16LE BOM has an "=encoding shiftjis" line).
If a Pod processor sees any command other than the ones listed above
(like "=head", or "=haed1", or "=stuff", or "=cuttlefish", or "=w123"),
that processor must by default treat this as an error. It must not
process the paragraph beginning with that command, must by default warn
of this as an error, and may abort the parse. A Pod parser may allow a
way for particular applications to add to the above list of known
commands, and to stipulate, for each additional command, whether
formatting codes should be processed.
Future versions of this specification may add additional commands.
PPoodd FFoorrmmaattttiinngg CCooddeess (Note that in previous drafts of this document and of perlpod, formatting codes were referred to as “interior sequences”, and this term may still be found in the documentation for Pod parsers, and in error messages from Pod processors.)
There are two syntaxes for formatting codes:
• A formatting code starts with a capital letter (just US-ASCII [A-Z])
followed by a "<", any number of characters, and ending with the
first matching ">". Examples:
That's what I<you> think!
What's C<CORE::dump()> for?
X<C<chmod> and C<unlink()> Under Different Operating Systems>
• A formatting code starts with a capital letter (just US-ASCII [A-Z])
followed by two or more "<"'s, one or more whitespace characters, any
number of characters, one or more whitespace characters, and ending
with the first matching sequence of two or more ">"'s, where the
number of ">"'s equals the number of "<"'s in the opening of this
formatting code. Examples:
That's what I<< you >> think!
C<<< open(X, ">>thing.dat") || die $! >>>
B<< $foo->bar(); >>
With this syntax, the whitespace character(s) after the "C<<<" and
before the ">>>" (or whatever letter) are _n_o_t renderable. They do not
signify whitespace, are merely part of the formatting codes
themselves. That is, these are all synonymous:
C<thing>
C<< thing >>
C<< thing >>
C<<< thing >>>
C«« #
thing
>>>>
and so on.
Finally, the multiple-angle-bracket form does _n_o_t alter the
interpretation of nested formatting codes, meaning that the following
four example lines are identical in meaning:
B<example: C<$a E<lt>=E<gt> $b>>
B<example: C<< $a <=> $b >>>
B<example: C<< $a E<lt>=E<gt> $b >>>
B<<< example: C<< $a E<lt>=E<gt> $b >> >>>
In parsing Pod, a notably tricky part is the correct parsing of
(potentially nested!) formatting codes. Implementors should consult the
code in the "parse_text" routine in Pod::Parser as an example of a
correct implementation.
"I<text>" -- italic text
See the brief discussion in "Formatting Codes" in perlpod.
"B<text>" -- bold text
See the brief discussion in "Formatting Codes" in perlpod.
"C<code>" -- code text
See the brief discussion in "Formatting Codes" in perlpod.
"F<filename>" -- style for filenames
See the brief discussion in "Formatting Codes" in perlpod.
"X<topic name>" -- an index entry
See the brief discussion in "Formatting Codes" in perlpod.
This code is unusual in that most formatters completely discard this
code and its content. Other formatters will render it with invisible
codes that can be used in building an index of the current document.
"Z<>" -- a null (zero-effect) formatting code
Discussed briefly in "Formatting Codes" in perlpod.
This code is unusual in that it should have no content. That is, a
processor may complain if it sees "Z<potatoes>". Whether or not it
complains, the _p_o_t_a_t_o_e_s text should ignored.
"L<name>" -- a hyperlink
The complicated syntaxes of this code are discussed at length in
"Formatting Codes" in perlpod, and implementation details are
discussed below, in "About L<...> Codes". Parsing the contents of
L<content> is tricky. Notably, the content has to be checked for
whether it looks like a URL, or whether it has to be split on literal
"|" and/or "/" (in the right order!), and so on, _b_e_f_o_r_e E<...> codes
are resolved.
"E<escape>" -- a character escape
See "Formatting Codes" in perlpod, and several points in "Notes on
Implementing Pod Processors".
"S<text>" -- text contains non-breaking spaces
This formatting code is syntactically simple, but semantically
complex. What it means is that each space in the printable content
of this code signifies a non-breaking space.
Consider:
C<$x ? $y : $z>
S<C<$x ? $y : $z>>
Both signify the monospace (c[ode] style) text consisting of "$x",
one space, "?", one space, ":", one space, "$z". The difference is
that in the latter, with the S code, those spaces are not "normal"
spaces, but instead are non-breaking spaces.
If a Pod processor sees any formatting code other than the ones listed
above (as in "N<...>", or "Q<...>", etc.), that processor must by default
treat this as an error. A Pod parser may allow a way for particular
applications to add to the above list of known formatting codes; a Pod
parser might even allow a way to stipulate, for each additional command,
whether it requires some form of special processing, as L<...> does.
Future versions of this specification may add additional formatting
codes.
Historical note: A few older Pod processors would not see a ">" as
closing a "C<" code, if the ">" was immediately preceded by a "-". This
was so that this:
C<$foo->bar>
would parse as equivalent to this:
C<$foo-E<gt>bar>
instead of as equivalent to a "C" formatting code containing only
"$foo-", and then a "bar>" outside the "C" formatting code. This problem
has since been solved by the addition of syntaxes like this:
C<< $foo->bar >>
Compliant parsers must not treat "->" as special.
Formatting codes absolutely cannot span paragraphs. If a code is opened
in one paragraph, and no closing code is found by the end of that
paragraph, the Pod parser must close that formatting code, and should
complain (as in "Unterminated I code in the paragraph starting at line
123: 'Time objects are not...'"). So these two paragraphs:
I<I told you not to do this!
Don't make me say it again!>
...must _n_o_t be parsed as two paragraphs in italics (with the I code
starting in one paragraph and starting in another.) Instead, the first
paragraph should generate a warning, but that aside, the above code must
parse as if it were:
I<I told you not to do this!>
Don't make me say it again!E<gt>
(In SGMLish jargon, all Pod commands are like block-level elements,
whereas all Pod formatting codes are like inline-level elements.)
NNootteess oonn IImmpplleemmeennttiinngg PPoodd PPrroocceessssoorrss The following is a long section of miscellaneous requirements and suggestions to do with Pod processing.
• Pod formatters should tolerate lines in verbatim blocks that are of
any length, even if that means having to break them (possibly several
times, for very long lines) to avoid text running off the side of the
page. Pod formatters may warn of such line-breaking. Such warnings
are particularly appropriate for lines are over 100 characters long,
which are usually not intentional.
• Pod parsers must recognize _a_l_l of the three well-known newline
formats: CR, LF, and CRLF. See perlport.
• Pod parsers should accept input lines that are of any length.
• Since Perl recognizes a Unicode Byte Order Mark at the start of files
as signaling that the file is Unicode encoded as in UTF-16 (whether
big-endian or little-endian) or UTF-8, Pod parsers should do the
same. Otherwise, the character encoding should be understood as
being UTF-8 if the first highbit byte sequence in the file seems
valid as a UTF-8 sequence, or otherwise as CP-1252 (earlier versions
of this specification used Latin-1 instead of CP-1252).
Future versions of this specification may specify how Pod can accept
other encodings. Presumably treatment of other encodings in Pod
parsing would be as in XML parsing: whatever the encoding declared by
a particular Pod file, content is to be stored in memory as Unicode
characters.
• The well known Unicode Byte Order Marks are as follows: if the file
begins with the two literal byte values 0xFE 0xFF, this is the BOM
for big-endian UTF-16. If the file begins with the two literal byte
value 0xFF 0xFE, this is the BOM for little-endian UTF-16. On an
ASCII platform, if the file begins with the three literal byte values
0xEF 0xBB 0xBF, this is the BOM for UTF-8. A mechanism portable to
EBCDIC platforms is to:
my $utf8_bom = "\x{FEFF}";
utf8::encode($utf8_bom);
• A naive, but often sufficient heuristic on ASCII platforms, for
testing the first highbit byte-sequence in a BOM-less file (whether
in code or in Pod!), to see whether that sequence is valid as UTF-8
(RFC 2279) is to check whether that the first byte in the sequence is
in the range 0xC2 - 0xFD _a_n_d whether the next byte is in the range
0x80 - 0xBF. If so, the parser may conclude that this file is in
UTF-8, and all highbit sequences in the file should be assumed to be
UTF-8. Otherwise the parser should treat the file as being in
CP-1252. (A better check, and which works on EBCDIC platforms as
well, is to pass a copy of the sequence to uuttff88::::ddeeccooddee(()) which
performs a full validity check on the sequence and returns TRUE if it
is valid UTF-8, FALSE otherwise. This function is always pre-loaded,
is fast because it is written in C, and will only get called at most
once, so you don't need to avoid it out of performance concerns.) In
the unlikely circumstance that the first highbit sequence in a truly
non-UTF-8 file happens to appear to be UTF-8, one can cater to our
heuristic (as well as any more intelligent heuristic) by prefacing
that line with a comment line containing a highbit sequence that is
clearly _n_o_t valid as UTF-8. A line consisting of simply "#", an
e-acute, and any non-highbit byte, is sufficient to establish this
file's encoding.
• Pod processors must treat a "=for [label] [content...]" paragraph as
meaning the same thing as a "=begin [label]" paragraph, content, and
an "=end [label]" paragraph. (The parser may conflate these two
constructs, or may leave them distinct, in the expectation that the
formatter will nevertheless treat them the same.)
• When rendering Pod to a format that allows comments (i.e., to nearly
any format other than plaintext), a Pod formatter must insert comment
text identifying its name and version number, and the name and
version numbers of any modules it might be using to process the Pod.
Minimal examples:
%% POD::Pod2PS v3.14159, using POD::Parser v1.92
<!-- Pod::HTML v3.14159, using POD::Parser v1.92 -->
{\doccomm generated by Pod::Tree::RTF 3.14159 using Pod::Tree 1.08}
.\" Pod::Man version 3.14159, using POD::Parser version 1.92
Formatters may also insert additional comments, including: the
release date of the Pod formatter program, the contact address for
the author(s) of the formatter, the current time, the name of input
file, the formatting options in effect, version of Perl used, etc.
Formatters may also choose to note errors/warnings as comments,
besides or instead of emitting them otherwise (as in messages to
STDERR, or "die"ing).
• Pod parsers _m_a_y emit warnings or error messages ("Unknown E code
E<zslig>!") to STDERR (whether through printing to STDERR, or
"warn"ing/"carp"ing, or "die"ing/"croak"ing), but _m_u_s_t allow
suppressing all such STDERR output, and instead allow an option for
reporting errors/warnings in some other way, whether by triggering a
callback, or noting errors in some attribute of the document object,
or some similarly unobtrusive mechanism -- or even by appending a
"Pod Errors" section to the end of the parsed form of the document.
• In cases of exceptionally aberrant documents, Pod parsers may abort
the parse. Even then, using "die"ing/"croak"ing is to be avoided;
where possible, the parser library may simply close the input file
and add text like "*** Formatting Aborted ***" to the end of the
(partial) in-memory document.
• In paragraphs where formatting codes (like E<...>, B<...>) are
understood (i.e., _n_o_t verbatim paragraphs, but _i_n_c_l_u_d_i_n_g ordinary
paragraphs, and command paragraphs that produce renderable text, like
"=head1"), literal whitespace should generally be considered
"insignificant", in that one literal space has the same meaning as
any (nonzero) number of literal spaces, literal newlines, and literal
tabs (as long as this produces no blank lines, since those would
terminate the paragraph). Pod parsers should compact literal
whitespace in each processed paragraph, but may provide an option for
overriding this (since some processing tasks do not require it), or
may follow additional special rules (for example, specially treating
period-space-space or period-newline sequences).
• Pod parsers should not, by default, try to coerce apostrophe (') and
quote (") into smart quotes (little 9's, 66's, 99's, etc), nor try to
turn backtick (`) into anything else but a single backtick character
(distinct from an open quote character!), nor "--" into anything but
two minus signs. They _m_u_s_t _n_e_v_e_r do any of those things to text in
C<...> formatting codes, and never _e_v_e_r to text in verbatim
paragraphs.
• When rendering Pod to a format that has two kinds of hyphens (-), one
that's a non-breaking hyphen, and another that's a breakable hyphen
(as in "object-oriented", which can be split across lines as
"object-", newline, "oriented"), formatters are encouraged to
generally translate "-" to non-breaking hyphen, but may apply
heuristics to convert some of these to breaking hyphens.
• Pod formatters should make reasonable efforts to keep words of Perl
code from being broken across lines. For example, "Foo::Bar" in some
formatting systems is seen as eligible for being broken across lines
as "Foo::" newline "Bar" or even "Foo::-" newline "Bar". This should
be avoided where possible, either by disabling all line-breaking in
mid-word, or by wrapping particular words with internal punctuation
in "don't break this across lines" codes (which in some formats may
not be a single code, but might be a matter of inserting non-breaking
zero-width spaces between every pair of characters in a word.)
• Pod parsers should, by default, expand tabs in verbatim paragraphs as
they are processed, before passing them to the formatter or other
processor. Parsers may also allow an option for overriding this.
• Pod parsers should, by default, remove newlines from the end of
ordinary and verbatim paragraphs before passing them to the
formatter. For example, while the paragraph you're reading now could
be considered, in Pod source, to end with (and contain) the
newline(s) that end it, it should be processed as ending with (and
containing) the period character that ends this sentence.
• Pod parsers, when reporting errors, should make some effort to report
an approximate line number ("Nested E<>'s in Paragraph #52, near line
633 of Thing/Foo.pm!"), instead of merely noting the paragraph number
("Nested E<>'s in Paragraph #52 of Thing/Foo.pm!"). Where this is
problematic, the paragraph number should at least be accompanied by
an excerpt from the paragraph ("Nested E<>'s in Paragraph #52 of
Thing/Foo.pm, which begins 'Read/write accessor for the C<interest
rate> attribute...'").
• Pod parsers, when processing a series of verbatim paragraphs one
after another, should consider them to be one large verbatim
paragraph that happens to contain blank lines. I.e., these two
lines, which have a blank line between them:
use Foo;
print Foo->VERSION
should be unified into one paragraph ("\tuse Foo;\n\n\tprint
Foo->VERSION") before being passed to the formatter or other
processor. Parsers may also allow an option for overriding this.
While this might be too cumbersome to implement in event-based Pod
parsers, it is straightforward for parsers that return parse trees.
• Pod formatters, where feasible, are advised to avoid splitting short
verbatim paragraphs (under twelve lines, say) across pages.
• Pod parsers must treat a line with only spaces and/or tabs on it as a
"blank line" such as separates paragraphs. (Some older parsers
recognized only two adjacent newlines as a "blank line" but would not
recognize a newline, a space, and a newline, as a blank line. This
is noncompliant behavior.)
• Authors of Pod formatters/processors should make every effort to
avoid writing their own Pod parser. There are already several in
CPAN, with a wide range of interface styles -- and one of them,
Pod::Simple, comes with modern versions of Perl.
• Characters in Pod documents may be conveyed either as literals, or by
number in E<n> codes, or by an equivalent mnemonic, as in E<eacute>
which is exactly equivalent to E<233>. The numbers are the
Latin1/Unicode values, even on EBCDIC platforms.
When referring to characters by using a E<n> numeric code, numbers in
the range 32-126 refer to those well known US-ASCII characters (also
defined there by Unicode, with the same meaning), which all Pod
formatters must render faithfully. Characters whose E<> numbers are
in the ranges 0-31 and 127-159 should not be used (neither as
literals, nor as E<number> codes), except for the literal byte-
sequences for newline (ASCII 13, ASCII 13 10, or ASCII 10), and tab
(ASCII 9). #
Numbers in the range 160-255 refer to Latin-1 characters (also
defined there by Unicode, with the same meaning). Numbers above 255
should be understood to refer to Unicode characters.
• Be warned that some formatters cannot reliably render characters
outside 32-126; and many are able to handle 32-126 and 160-255, but
nothing above 255.
• Besides the well-known "E<lt>" and "E<gt>" codes for less-than and
greater-than, Pod parsers must understand "E<sol>" for "/" (solidus,
slash), and "E<verbar>" for "|" (vertical bar, pipe). Pod parsers
should also understand "E<lchevron>" and "E<rchevron>" as legacy
codes for characters 171 and 187, i.e., "left-pointing double angle
quotation mark" = "left pointing guillemet" and "right-pointing
double angle quotation mark" = "right pointing guillemet". (These
look like little "<<" and ">>", and they are now preferably expressed
with the HTML/XHTML codes "E<laquo>" and "E<raquo>".)
• Pod parsers should understand all "E<html>" codes as defined in the
entity declarations in the most recent XHTML specification at
"www.W3.org". Pod parsers must understand at least the entities that
define characters in the range 160-255 (Latin-1). Pod parsers, when
faced with some unknown "E<_i_d_e_n_t_i_f_i_e_r>" code, shouldn't simply
replace it with nullstring (by default, at least), but may pass it
through as a string consisting of the literal characters E, less-
than, _i_d_e_n_t_i_f_i_e_r, greater-than. Or Pod parsers may offer the
alternative option of processing such unknown "E<_i_d_e_n_t_i_f_i_e_r>" codes
by firing an event especially for such codes, or by adding a special
node-type to the in-memory document tree. Such "E<_i_d_e_n_t_i_f_i_e_r>" may
have special meaning to some processors, or some processors may
choose to add them to a special error report.
• Pod parsers must also support the XHTML codes "E<quot>" for character
34 (doublequote, "), "E<amp>" for character 38 (ampersand, &), and
"E<apos>" for character 39 (apostrophe, ').
• Note that in all cases of "E<whatever>", _w_h_a_t_e_v_e_r (whether an
htmlname, or a number in any base) must consist only of alphanumeric
characters -- that is, _w_h_a_t_e_v_e_r must match "m/\A\w+\z/". So
"E< 0 1 2 3 >" is invalid, because it contains spaces, which aren't
alphanumeric characters. This presumably does not _n_e_e_d special
treatment by a Pod processor; " 0 1 2 3 " doesn't look like a number
in any base, so it would presumably be looked up in the table of
HTML-like names. Since there isn't (and cannot be) an HTML-like
entity called " 0 1 2 3 ", this will be treated as an error.
However, Pod processors may treat "E< 0 1 2 3 >" or "E<e-acute>" as
_s_y_n_t_a_c_t_i_c_a_l_l_y invalid, potentially earning a different error message
than the error message (or warning, or event) generated by a merely
unknown (but theoretically valid) htmlname, as in "E<qacute>" [sic].
However, Pod parsers are not required to make this distinction.
• Note that E<number> _m_u_s_t _n_o_t be interpreted as simply "codepoint
_n_u_m_b_e_r in the current/native character set". It always means only
"the character represented by codepoint _n_u_m_b_e_r in Unicode." (This is
identical to the semantics of &#_n_u_m_b_e_r; in XML.)
This will likely require many formatters to have tables mapping from
treatable Unicode codepoints (such as the "\xE9" for the e-acute
character) to the escape sequences or codes necessary for conveying
such sequences in the target output format. A converter to *roff
would, for example know that "\xE9" (whether conveyed literally, or
via a E<...> sequence) is to be conveyed as "e\\*'". Similarly, a
program rendering Pod in a Mac OS application window, would
presumably need to know that "\xE9" maps to codepoint 142 in MacRoman
encoding that (at time of writing) is native for Mac OS. Such
Unicode2whatever mappings are presumably already widely available for
common output formats. (Such mappings may be incomplete!
Implementers are not expected to bend over backwards in an attempt to
render Cherokee syllabics, Etruscan runes, Byzantine musical symbols,
or any of the other weird things that Unicode can encode.) And if a
Pod document uses a character not found in such a mapping, the
formatter should consider it an unrenderable character.
• If, surprisingly, the implementor of a Pod formatter can't find a
satisfactory pre-existing table mapping from Unicode characters to
escapes in the target format (e.g., a decent table of Unicode
characters to *roff escapes), it will be necessary to build such a
table. If you are in this circumstance, you should begin with the
characters in the range 0x00A0 - 0x00FF, which is mostly the heavily
used accented characters. Then proceed (as patience permits and
fastidiousness compels) through the characters that the (X)HTML
standards groups judged important enough to merit mnemonics for.
These are declared in the (X)HTML specifications at the www.W3.org
site. At time of writing (September 2001), the most recent entity
declaration files are:
http://www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
http://www.w3.org/TR/xhtml1/DTD/xhtml-special.ent
http://www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent
Then you can progress through any remaining notable Unicode
characters in the range 0x2000-0x204D (consult the character tables
at www.unicode.org), and whatever else strikes your fancy. For
example, in _x_h_t_m_l_-_s_y_m_b_o_l_._e_n_t, there is the entry:
<!ENTITY infin "∞"> <!-- infinity, U+221E ISOtech -->
While the mapping "infin" to the character "\x{221E}" will
(hopefully) have been already handled by the Pod parser, the presence
of the character in this file means that it's reasonably important
enough to include in a formatter's table that maps from notable
Unicode characters to the codes necessary for rendering them. So for
a Unicode-to-*roff mapping, for example, this would merit the entry:
"\x{221E}" => '\(in',
It is eagerly hoped that in the future, increasing numbers of formats
(and formatters) will support Unicode characters directly (as (X)HTML
does with "∞", "∞", or "∞"), reducing the need for
idiosyncratic mappings of Unicode-to-_m_y___e_s_c_a_p_e_s.
• It is up to individual Pod formatter to display good judgement when
confronted with an unrenderable character (which is distinct from an
unknown E<thing> sequence that the parser couldn't resolve to
anything, renderable or not). It is good practice to map Latin
letters with diacritics (like "E<eacute>"/"E<233>") to the
corresponding unaccented US-ASCII letters (like a simple character
101, "e"), but clearly this is often not feasible, and an
unrenderable character may be represented as "?", or the like. In
attempting a sane fallback (as from E<233> to "e"), Pod formatters
may use the %Latin1Code_to_fallback table in Pod::Escapes, or
Text::Unidecode, if available.
For example, this Pod text:
magic is enabled if you set C<$Currency> to 'E<euro>'.
may be rendered as: "magic is enabled if you set $Currency to '_?'" or
as "magic is enabled if you set $Currency to '[[eeuurroo]]'", or as "magic
is enabled if you set $Currency to '[x20AC]', etc.
A Pod formatter may also note, in a comment or warning, a list of
what unrenderable characters were encountered.
• E<...> may freely appear in any formatting code (other than in
another E<...> or in an Z<>). That is, "X<The E<euro>1,000,000
Solution>" is valid, as is "L<The E<euro>1,000,000
Solution|Million::Euros>".
• Some Pod formatters output to formats that implement non-breaking
spaces as an individual character (which I'll call "NBSP"), and
others output to formats that implement non-breaking spaces just as
spaces wrapped in a "don't break this across lines" code. Note that
at the level of Pod, both sorts of codes can occur: Pod can contain a
NBSP character (whether as a literal, or as a "E<160>" or "E<nbsp>"
code); and Pod can contain "S<foo I<bar> baz>" codes, where "mere
spaces" (character 32) in such codes are taken to represent non-
breaking spaces. Pod parsers should consider supporting the optional
parsing of "S<foo I<bar> baz>" as if it were "foo_N_B_S_PI<bar>_N_B_S_Pbaz",
and, going the other way, the optional parsing of groups of words
joined by NBSP's as if each group were in a S<...> code, so that
formatters may use the representation that maps best to what the
output format demands.
• Some processors may find that the "S<...>" code is easiest to
implement by replacing each space in the parse tree under the content
of the S, with an NBSP. But note: the replacement should apply _n_o_t
to spaces in _a_l_l text, but _o_n_l_y to spaces in _p_r_i_n_t_a_b_l_e text. (This
distinction may or may not be evident in the particular tree/event
model implemented by the Pod parser.) For example, consider this
unusual case:
S<L</Autoloaded Functions>>
This means that the space in the middle of the visible link text must
not be broken across lines. In other words, it's the same as this:
L<"AutoloadedE<160>Functions"/Autoloaded Functions>
However, a misapplied space-to-NBSP replacement could (wrongly)
produce something equivalent to this:
L<"AutoloadedE<160>Functions"/AutoloadedE<160>Functions>
...which is almost definitely not going to work as a hyperlink
(assuming this formatter outputs a format supporting hypertext).
Formatters may choose to just not support the S format code,
especially in cases where the output format simply has no NBSP
character/code and no code for "don't break this stuff across lines".
• Besides the NBSP character discussed above, implementors are reminded
of the existence of the other "special" character in Latin-1, the
"soft hyphen" character, also known as "discretionary hyphen", i.e.
"E<173>" = "E<0xAD>" = "E<shy>"). This character expresses an
optional hyphenation point. That is, it normally renders as nothing,
but may render as a "-" if a formatter breaks the word at that point.
Pod formatters should, as appropriate, do one of the following: 1)
render this with a code with the same meaning (e.g., "\-" in RTF), 2)
pass it through in the expectation that the formatter understands
this character as such, or 3) delete it.
For example:
sigE<shy>action
manuE<shy>script
JarkE<shy>ko HieE<shy>taE<shy>nieE<shy>mi
These signal to a formatter that if it is to hyphenate "sigaction" or
"manuscript", then it should be done as "sig-_[_l_i_n_e_b_r_e_a_k_]action" or
"manu-_[_l_i_n_e_b_r_e_a_k_]script" (and if it doesn't hyphenate it, then the
"E<shy>" doesn't show up at all). And if it is to hyphenate "Jarkko"
and/or "Hietaniemi", it can do so only at the points where there is a
"E<shy>" code.
In practice, it is anticipated that this character will not be used
often, but formatters should either support it, or delete it.
• If you think that you want to add a new command to Pod (like, say, a
"=biblio" command), consider whether you could get the same effect
with a for or begin/end sequence: "=for biblio ..." or "=begin
biblio" ... "=end biblio". Pod processors that don't understand
"=for biblio", etc, will simply ignore it, whereas they may complain
loudly if they see "=biblio".
• Throughout this document, "Pod" has been the preferred spelling for
the name of the documentation format. One may also use "POD" or
"pod". For the documentation that is (typically) in the Pod format,
you may use "pod", or "Pod", or "POD". Understanding these
distinctions is useful; but obsessing over how to spell them, usually
is not.
AAbboouutt LL<<......>> CCooddeess As you can tell from a glance at perlpod, the L<…> code is the most complex of the Pod formatting codes. The points below will hopefully clarify what it means and how processors should deal with it.
• In parsing an L<...> code, Pod parsers must distinguish at least four
attributes:
First:
The link-text. If there is none, this must be "undef". (E.g.,
in "L<Perl Functions|perlfunc>", the link-text is "Perl
Functions". In "L<Time::HiRes>" and even "L<|Time::HiRes>",
there is no link text. Note that link text may contain
formatting.)
Second:
The possibly inferred link-text; i.e., if there was no real link
text, then this is the text that we'll infer in its place.
(E.g., for "L<Getopt::Std>", the inferred link text is
"Getopt::Std".)
Third:
The name or URL, or "undef" if none. (E.g., in "L<Perl
Functions|perlfunc>", the name (also sometimes called the page)
is "perlfunc". In "L</CAVEATS>", the name is "undef".)
Fourth:
The section (AKA "item" in older perlpods), or "undef" if none.
E.g., in "L<Getopt::Std/DESCRIPTION>", "DESCRIPTION" is the
section. (Note that this is not the same as a manpage section
like the "5" in "man 5 crontab". "Section Foo" in the Pod sense
means the part of the text that's introduced by the heading or
item whose text is "Foo".)
Pod parsers may also note additional attributes including:
Fifth:
A flag for whether item 3 (if present) is a URL (like
"http://lists.perl.org" is), in which case there should be no
section attribute; a Pod name (like "perldoc" and "Getopt::Std"
are); or possibly a man page name (like "ccrroonnttaabb(5)" is).
Sixth:
The raw original L<...> content, before text is split on "|",
"/", etc, and before E<...> codes are expanded.
(The above were numbered only for concise reference below. It is not
a requirement that these be passed as an actual list or array.)
For example:
L<Foo::Bar>
=> undef, # link text
"Foo::Bar", # possibly inferred link text
"Foo::Bar", # name
undef, # section
'pod', # what sort of link
"Foo::Bar" # original content
L<Perlport's section on NL's|perlport/Newlines>
=> "Perlport's section on NL's", # link text
"Perlport's section on NL's", # possibly inferred link text
"perlport", # name
"Newlines", # section
'pod', # what sort of link
"Perlport's section on NL's|perlport/Newlines"
# original content
L<perlport/Newlines>
=> undef, # link text
'"Newlines" in perlport', # possibly inferred link text
"perlport", # name
"Newlines", # section
'pod', # what sort of link
"perlport/Newlines" # original content
L<crontab(5)/"DESCRIPTION">
=> undef, # link text
'"DESCRIPTION" in crontab(5)', # possibly inferred link text
"crontab(5)", # name
"DESCRIPTION", # section
'man', # what sort of link
'crontab(5)/"DESCRIPTION"' # original content
L</Object Attributes>
=> undef, # link text
'"Object Attributes"', # possibly inferred link text
undef, # name
"Object Attributes", # section
'pod', # what sort of link
"/Object Attributes" # original content
L<https://www.perl.org/>
=> undef, # link text
"https://www.perl.org/", # possibly inferred link text
"https://www.perl.org/", # name
undef, # section
'url', # what sort of link
"https://www.perl.org/" # original content
L<Perl.org|https://www.perl.org/>
=> "Perl.org", # link text
"https://www.perl.org/", # possibly inferred link text
"https://www.perl.org/", # name
undef, # section
'url', # what sort of link
"Perl.org|https://www.perl.org/" # original content
Note that you can distinguish URL-links from anything else by the
fact that they match "m/\A\w+:[^:\s]\S*\z/". So
"L<http://www.perl.com>" is a URL, but "L<HTTP::Response>" isn't.
• In case of L<...> codes with no "text|" part in them, older
formatters have exhibited great variation in actually displaying the
link or cross reference. For example, L<ccrroonnttaabb(5)> would render as
"the crontab(5) manpage", or "in the crontab(5) manpage" or just
"crontab(5)".
Pod processors must now treat "text|"-less links as follows:
L<name> => L<name|name>
L</section> => L<"section"|/section>
L<name/section> => L<"section" in name|name/section>
• Note that section names might contain markup. I.e., if a section
starts with:
=head2 About the C<-M> Operator
or with:
=item About the C<-M> Operator
then a link to it would look like this:
L<somedoc/About the C<-M> Operator>
Formatters may choose to ignore the markup for purposes of resolving
the link and use only the renderable characters in the section name,
as in:
<h1><a name="About_the_-M_Operator">About the <code>-M</code>
Operator</h1>
...
<a href="somedoc#About_the_-M_Operator">About the <code>-M</code>
Operator" in somedoc</a>
• Previous versions of perlpod distinguished "L<name/"section">" links
from "L<name/item>" links (and their targets). These have been
merged syntactically and semantically in the current specification,
and _s_e_c_t_i_o_n can refer either to a "=head_n Heading Content" command or
to a "=item Item Content" command. This specification does not
specify what behavior should be in the case of a given document
having several things all seeming to produce the same _s_e_c_t_i_o_n
identifier (e.g., in HTML, several things all producing the same
_a_n_c_h_o_r_n_a_m_e in <a name="_a_n_c_h_o_r_n_a_m_e">...</a> elements). Where Pod
processors can control this behavior, they should use the first such
anchor. That is, "L<Foo/Bar>" refers to the _f_i_r_s_t "Bar" section in
Foo.
But for some processors/formats this cannot be easily controlled; as
with the HTML example, the behavior of multiple ambiguous <a
name="_a_n_c_h_o_r_n_a_m_e">...</a> is most easily just left up to browsers to
decide.
• In a "L<text|...>" code, text may contain formatting codes for
formatting or for E<...> escapes, as in:
L<B<ummE<234>stuff>|...>
For "L<...>" codes without a "name|" part, only "E<...>" and "Z<>"
codes may occur. That is, authors should not use ""L<B<Foo::Bar>>"".
Note, however, that formatting codes and Z<>'s can occur in any and
all parts of an L<...> (i.e., in _n_a_m_e, _s_e_c_t_i_o_n, _t_e_x_t, and _u_r_l).
Authors must not nest L<...> codes. For example, "L<The L<Foo::Bar>
man page>" should be treated as an error.
• Note that Pod authors may use formatting codes inside the "text" part
of "L<text|name>" (and so on for L<text|/"sec">).
In other words, this is valid:
Go read L<the docs on C<$.>|perlvar/"$.">
Some output formats that do allow rendering "L<...>" codes as
hypertext, might not allow the link-text to be formatted; in that
case, formatters will have to just ignore that formatting.
• At time of writing, "L<name>" values are of two types: either the
name of a Pod page like "L<Foo::Bar>" (which might be a real Perl
module or program in an @INC / PATH directory, or a .pod file in
those places); or the name of a Unix man page, like "L<crontab(5)>".
In theory, "L<chmod>" is ambiguous between a Pod page called "chmod",
or the Unix man page "chmod" (in whatever man-section). However, the
presence of a string in parens, as in "ccrroonnttaabb(5)", is sufficient to
signal that what is being discussed is not a Pod page, and so is
presumably a Unix man page. The distinction is of no importance to
many Pod processors, but some processors that render to hypertext
formats may need to distinguish them in order to know how to render a
given "L<foo>" code.
• Previous versions of perlpod allowed for a "L<section>" syntax (as in
"L<Object Attributes>"), which was not easily distinguishable from
"L<name>" syntax and for "L<"section">" which was only slightly less
ambiguous. This syntax is no longer in the specification, and has
been replaced by the "L</section>" syntax (where the slash was
formerly optional). Pod parsers should tolerate the "L<"section">"
syntax, for a while at least. The suggested heuristic for
distinguishing "L<section>" from "L<name>" is that if it contains any
whitespace, it's a _s_e_c_t_i_o_n. Pod processors should warn about this
being deprecated syntax.
AAbboouutt ==oovveerr......==bbaacckk RReeggiioonnss “=over”…"=back" regions are used for various kinds of list-like structures. (I use the term “region” here simply as a collective term for everything from the “=over” to the matching “=back”.)
• The non-zero numeric _i_n_d_e_n_t_l_e_v_e_l in "=over _i_n_d_e_n_t_l_e_v_e_l" ... "=back"
is used for giving the formatter a clue as to how many "spaces" (ems,
or roughly equivalent units) it should tab over, although many
formatters will have to convert this to an absolute measurement that
may not exactly match with the size of spaces (or M's) in the
document's base font. Other formatters may have to completely ignore
the number. The lack of any explicit _i_n_d_e_n_t_l_e_v_e_l parameter is
equivalent to an _i_n_d_e_n_t_l_e_v_e_l value of 4. Pod processors may complain
if _i_n_d_e_n_t_l_e_v_e_l is present but is not a positive number matching
"m/\A(\d*\.)?\d+\z/".
• Authors of Pod formatters are reminded that "=over" ... "=back" may
map to several different constructs in your output format. For
example, in converting Pod to (X)HTML, it can map to any of
<ul>...</ul>, <ol>...</ol>, <dl>...</dl>, or
<blockquote>...</blockquote>. Similarly, "=item" can map to <li> or
<dt>.
• Each "=over" ... "=back" region should be one of the following:
• An "=over" ... "=back" region containing only "=item *" commands,
each followed by some number of ordinary/verbatim paragraphs,
other nested "=over" ... "=back" regions, "=for..." paragraphs,
and "=begin"..."=end" regions.
(Pod processors must tolerate a bare "=item" as if it were "=item
*".) Whether "*" is rendered as a literal asterisk, an "o", or
as some kind of real bullet character, is left up to the Pod
formatter, and may depend on the level of nesting.
• An "=over" ... "=back" region containing only
"m/\A=item\s+\d+\.?\s*\z/" paragraphs, each one (or each group of
them) followed by some number of ordinary/verbatim paragraphs,
other nested "=over" ... "=back" regions, "=for..." paragraphs,
and/or "=begin"..."=end" codes. Note that the numbers must start
at 1 in each section, and must proceed in order and without
skipping numbers.
(Pod processors must tolerate lines like "=item 1" as if they
were "=item 1.", with the period.)
• An "=over" ... "=back" region containing only "=item [text]"
commands, each one (or each group of them) followed by some
number of ordinary/verbatim paragraphs, other nested "=over" ...
"=back" regions, or "=for..." paragraphs, and "=begin"..."=end"
regions.
The "=item [text]" paragraph should not match
"m/\A=item\s+\d+\.?\s*\z/" or "m/\A=item\s+\*\s*\z/", nor should
it match just "m/\A=item\s*\z/".
• An "=over" ... "=back" region containing no "=item" paragraphs at
all, and containing only some number of ordinary/verbatim
paragraphs, and possibly also some nested "=over" ... "=back"
regions, "=for..." paragraphs, and "=begin"..."=end" regions.
Such an itemless "=over" ... "=back" region in Pod is equivalent
in meaning to a "<blockquote>...</blockquote>" element in HTML.
Note that with all the above cases, you can determine which type of
"=over" ... "=back" you have, by examining the first (non-"=cut",
non-"=pod") Pod paragraph after the "=over" command.
• Pod formatters _m_u_s_t tolerate arbitrarily large amounts of text in the
"=item _t_e_x_t_._._." paragraph. In practice, most such paragraphs are
short, as in:
=item For cutting off our trade with all parts of the world
But they may be arbitrarily long:
=item For transporting us beyond seas to be tried for pretended
offenses
=item He is at this time transporting large armies of foreign
mercenaries to complete the works of death, desolation and
tyranny, already begun with circumstances of cruelty and perfidy
scarcely paralleled in the most barbarous ages, and totally
unworthy the head of a civilized nation.
• Pod processors should tolerate "=item *" / "=item _n_u_m_b_e_r" commands
with no accompanying paragraph. The middle item is an example:
=over
=item 1
Pick up dry cleaning.
=item 2
=item 3
Stop by the store. Get Abba Zabas, Stoli, and cheap lawn chairs.
=back
• No "=over" ... "=back" region can contain headings. Processors may
treat such a heading as an error.
• Note that an "=over" ... "=back" region should have some content.
That is, authors should not have an empty region like this:
=over
=back
Pod processors seeing such a contentless "=over" ... "=back" region,
may ignore it, or may report it as an error.
• Processors must tolerate an "=over" list that goes off the end of the
document (i.e., which has no matching "=back"), but they may warn
about such a list.
• Authors of Pod formatters should note that this construct:
=item Neque
=item Porro
=item Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
=item Ut Enim
is semantically ambiguous, in a way that makes formatting decisions a
bit difficult. On the one hand, it could be mention of an item
"Neque", mention of another item "Porro", and mention of another item
"Quisquam Est", with just the last one requiring the explanatory
paragraph "Qui dolorem ipsum quia dolor..."; and then an item "Ut
Enim". In that case, you'd want to format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
But it could equally well be a discussion of three (related or
equivalent) items, "Neque", "Porro", and "Quisquam Est", followed by
a paragraph explaining them all, and then a new item "Ut Enim". In
that case, you'd probably want to format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
But (for the foreseeable future), Pod does not provide any way for
Pod authors to distinguish which grouping is meant by the above
"=item"-cluster structure. So formatters should format it like so:
Neque
Porro
Quisquam Est
Qui dolorem ipsum quia dolor sit amet, consectetur, adipisci
velit, sed quia non numquam eius modi tempora incidunt ut
labore et dolore magnam aliquam quaerat voluptatem.
Ut Enim
That is, there should be (at least roughly) equal spacing between
items as between paragraphs (although that spacing may well be less
than the full height of a line of text). This leaves it to the
reader to use (con)textual cues to figure out whether the "Qui
dolorem ipsum..." paragraph applies to the "Quisquam Est" item or to
all three items "Neque", "Porro", and "Quisquam Est". While not an
ideal situation, this is preferable to providing formatting cues that
may be actually contrary to the author's intent.
AAbboouutt DDaattaa PPaarraaggrraapphhss aanndd “”==bbeeggiinn//==eenndd"" RReeggiioonnss Data paragraphs are typically used for inlining non-Pod data that is to be used (typically passed through) when rendering the document to a specific format:
=begin rtf
\par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
=end rtf
The exact same effect could, incidentally, be achieved with a single
"=for" paragraph:
=for rtf \par{\pard\qr\sa4500{\i Printed\~\chdate\~\chtime}\par}
(Although that is not formally a data paragraph, it has the same meaning
as one, and Pod parsers may parse it as one.)
Another example of a data paragraph:
=begin html
I like <em>PIE</em>!
<hr>Especially pecan pie!
=end html
If these were ordinary paragraphs, the Pod parser would try to expand the
"E</em>" (in the first paragraph) as a formatting code, just like "E<lt>"
or "E<eacute>". But since this is in a "=begin _i_d_e_n_t_i_f_i_e_r"..."=end
_i_d_e_n_t_i_f_i_e_r" region _a_n_d the identifier "html" doesn't begin have a ":"
prefix, the contents of this region are stored as data paragraphs,
instead of being processed as ordinary paragraphs (or if they began with
a spaces and/or tabs, as verbatim paragraphs).
As a further example: At time of writing, no "biblio" identifier is
supported, but suppose some processor were written to recognize it as a
way of (say) denoting a bibliographic reference (necessarily containing
formatting codes in ordinary paragraphs). The fact that "biblio"
paragraphs were meant for ordinary processing would be indicated by
prefacing each "biblio" identifier with a colon:
=begin :biblio
Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
Programs.> Prentice-Hall, Englewood Cliffs, NJ.
=end :biblio
This would signal to the parser that paragraphs in this begin...end
region are subject to normal handling as ordinary/verbatim paragraphs
(while still tagged as meant only for processors that understand the
"biblio" identifier). The same effect could be had with:
=for :biblio
Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
Programs.> Prentice-Hall, Englewood Cliffs, NJ.
The ":" on these identifiers means simply "process this stuff normally,
even though the result will be for some special target". I suggest that
parser APIs report "biblio" as the target identifier, but also report
that it had a ":" prefix. (And similarly, with the above "html", report
"html" as the target identifier, and note the _l_a_c_k of a ":" prefix.)
Note that a "=begin _i_d_e_n_t_i_f_i_e_r"..."=end _i_d_e_n_t_i_f_i_e_r" region where
_i_d_e_n_t_i_f_i_e_r begins with a colon, _c_a_n contain commands. For example:
=begin :biblio
Wirth's classic is available in several editions, including:
=for comment
hm, check abebooks.com for how much used copies cost.
=over
=item
Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
Teubner, Stuttgart. [Yes, it's in German.]
=item
Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
Programs.> Prentice-Hall, Englewood Cliffs, NJ.
=back
=end :biblio
Note, however, a "=begin _i_d_e_n_t_i_f_i_e_r"..."=end _i_d_e_n_t_i_f_i_e_r" region where
_i_d_e_n_t_i_f_i_e_r does _n_o_t begin with a colon, should not directly contain
"=head1" ... "=head4" commands, nor "=over", nor "=back", nor "=item".
For example, this may be considered invalid:
=begin somedata
This is a data paragraph.
=head1 Don't do this!
This is a data paragraph too.
=end somedata
A Pod processor may signal that the above (specifically the "=head1"
paragraph) is an error. Note, however, that the following should _n_o_t be
treated as an error:
=begin somedata
This is a data paragraph.
=cut
# Yup, this isn't Pod anymore.
sub excl { (rand() > .5) ? "hoo!" : "hah!" }
=pod
This is a data paragraph too.
=end somedata
And this too is valid:
=begin someformat
This is a data paragraph.
And this is a data paragraph.
=begin someotherformat
This is a data paragraph too.
And this is a data paragraph too.
=begin :yetanotherformat
=head2 This is a command paragraph!
This is an ordinary paragraph!
And this is a verbatim paragraph!
=end :yetanotherformat
=end someotherformat
Another data paragraph!
=end someformat
The contents of the above "=begin :yetanotherformat" ... "=end
:yetanotherformat" region _a_r_e_n_'_t data paragraphs, because the immediately
containing region's identifier (":yetanotherformat") begins with a colon.
In practice, most regions that contain data paragraphs will contain _o_n_l_y
data paragraphs; however, the above nesting is syntactically valid as
Pod, even if it is rare. However, the handlers for some formats, like
"html", will accept only data paragraphs, not nested regions; and they
may complain if they see (targeted for them) nested regions, or commands,
other than "=end", "=pod", and "=cut".
Also consider this valid structure:
=begin :biblio
Wirth's classic is available in several editions, including:
=over
=item
Wirth, Niklaus. 1975. I<Algorithmen und Datenstrukturen.>
Teubner, Stuttgart. [Yes, it's in German.]
=item
Wirth, Niklaus. 1976. I<Algorithms + Data Structures =
Programs.> Prentice-Hall, Englewood Cliffs, NJ.
=back
Buy buy buy!
=begin html
<img src='wirth_spokesmodeling_book.png'>
<hr>
=end html
Now now now!
=end :biblio
There, the "=begin html"..."=end html" region is nested inside the larger
"=begin :biblio"..."=end :biblio" region. Note that the content of the
"=begin html"..."=end html" region is data paragraph(s), because the
immediately containing region's identifier ("html") _d_o_e_s_n_'_t begin with a
colon.
Pod parsers, when processing a series of data paragraphs one after
another (within a single region), should consider them to be one large
data paragraph that happens to contain blank lines. So the content of
the above "=begin html"..."=end html" _m_a_y be stored as two data
paragraphs (one consisting of "<img
src='wirth_spokesmodeling_book.png'>\n" and another consisting of
"<hr>\n"), but _s_h_o_u_l_d be stored as a single data paragraph (consisting of
"<img src='wirth_spokesmodeling_book.png'>\n\n<hr>\n").
Pod processors should tolerate empty "=begin _s_o_m_e_t_h_i_n_g"..."=end
_s_o_m_e_t_h_i_n_g" regions, empty "=begin :_s_o_m_e_t_h_i_n_g"..."=end :_s_o_m_e_t_h_i_n_g"
regions, and contentless "=for _s_o_m_e_t_h_i_n_g" and "=for :_s_o_m_e_t_h_i_n_g"
paragraphs. I.e., these should be tolerated:
=for html
=begin html
=end html
=begin :biblio
=end :biblio
Incidentally, note that there's no easy way to express a data paragraph
starting with something that looks like a command. Consider:
=begin stuff
=shazbot
=end stuff
There, "=shazbot" will be parsed as a Pod command "shazbot", not as a
data paragraph "=shazbot\n". However, you can express a data paragraph
consisting of "=shazbot\n" using this code:
=for stuff =shazbot
The situation where this is necessary, is presumably quite rare.
Note that =end commands must match the currently open =begin command.
That is, they must properly nest. For example, this is valid:
=begin outer
X #
=begin inner
Y #
=end inner
Z #
=end outer
while this is invalid:
=begin outer
X #
=begin inner
Y #
=end outer
Z #
=end inner
This latter is improper because when the "=end outer" command is seen,
the currently open region has the formatname "inner", not "outer". (It
just happens that "outer" is the format name of a higher-up region.)
This is an error. Processors must by default report this as an error,
and may halt processing the document containing that error. A corollary
of this is that regions cannot "overlap". That is, the latter block above
does not represent a region called "outer" which contains X and Y,
overlapping a region called "inner" which contains Y and Z. But because
it is invalid (as all apparently overlapping regions would be), it
doesn't represent that, or anything at all.
Similarly, this is invalid:
=begin thing
=end hting
This is an error because the region is opened by "thing", and the "=end"
tries to close "hting" [sic].
This is also invalid:
=begin thing
=end
This is invalid because every "=end" command must have a formatname
parameter.
SSEEEE AALLSSOO #
perlpod, "PODs: Embedded Documentation" in perlsyn, podchecker
AAUUTTHHOORR #
Sean M. Burke
perl v5.36.3 2023-02-15 PERLPODSPEC(1)