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diff --git a/Master/tlpkg/tlperl0/lib/pods/perlpodspec.pod b/Master/tlpkg/tlperl0/lib/pods/perlpodspec.pod new file mode 100755 index 00000000000..8973a7080cd --- /dev/null +++ b/Master/tlpkg/tlperl0/lib/pods/perlpodspec.pod @@ -0,0 +1,1899 @@ + +=head1 NAME + +perlpodspec - Plain Old Documentation: format specification and notes + +=head1 DESCRIPTION + +This document is detailed notes on the Pod markup language. Most +people will only have to read L<perlpod|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 I<nice> if X did Y" versus "it wouldn't +really I<bother> 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 I<not> do Y. I often phrase this as +"the parser should, by default, do Y." This doesn't I<require> +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 I<may> be provided. + +=head1 Pod Definitions + +Pod is embedded in files, typically Perl source files -- although you +can write a file that's nothing but Pod. + +A B<line> in a file consists of zero or more non-newline characters, +terminated by either a newline or the end of the file. + +A B<newline sequence> 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 B<blank line> 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 B<non-blank line> is a line containing one or more characters other +than space or tab (and terminated by a newline or end-of-file). + +(I<Note:> 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 I<no characters at all>, +terminated by a newline.) + +B<Whitespace> 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 "EE<lt>32>", which is a formatting +code that I<denotes> a whitespace character.) + +A B<Pod parser> is a module meant for parsing Pod (regardless of +whether this involves calling callbacks or building a parse tree or +directly formatting it). A B<Pod formatter> (or B<Pod translator>) +is a module or program that converts Pod to some other format (HTML, +plaintext, TeX, PostScript, RTF). A B<Pod processor> 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 B<Pod blocks>. A Pod block starts with a +line that matches <m/\A=[a-zA-Z]/>, and continues up to the next line +that matches C<m/\A=cut/> -- or up to the end of the file, if there is +no C<m/\A=cut/> line. + +=for comment + The current perlsyn says: + [beginquote] + Note that pod translators should look at only paragraphs beginning + with a pod directive (it makes parsing easier), whereas the compiler + actually knows to look for pod escapes even in the middle of a + paragraph. This means that the following secret stuff will be ignored + by both the compiler and the translators. + $a=3; + =secret stuff + warn "Neither POD nor CODE!?" + =cut back + print "got $a\n"; + You probably shouldn't rely upon the warn() being podded out forever. + Not all pod translators are well-behaved in this regard, and perhaps + the compiler will become pickier. + [endquote] + I think that those paragraphs should just be removed; paragraph-based + parsing seems to have been largely abandoned, because of the hassle + with non-empty blank lines messing up what people meant by "paragraph". + Even if the "it makes parsing easier" bit were especially true, + it wouldn't be worth the confusion of having perl and pod2whatever + actually disagree on what can constitute a Pod block. + +Within a Pod block, there are B<Pod paragraphs>. 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: + +=over + +=item * + +A command paragraph (also called a "directive"). The first line of +this paragraph must match C<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 + +I<Some> command paragraphs allow formatting codes in their content +(i.e., after the part that matches C<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 CE<lt>use strict;>?" that it +would to an ordinary paragraph -- i.e., formatting codes (like +"CE<lt>...>") are parsed and presumably formatted appropriately, and +whitespace in the form of literal spaces and/or tabs is not +significant. + +=item * + +A B<verbatim paragraph>. The first line of this paragraph must be a +literal space or tab, and this paragraph must not be inside a "=begin +I<identifier>", ... "=end I<identifier>" sequence unless +"I<identifier>" begins with a colon (":"). That is, if a paragraph +starts with a literal space or tab, but I<is> inside a +"=begin I<identifier>", ... "=end I<identifier>" region, then it's +a data paragraph, unless "I<identifier>" begins with a colon. + +Whitespace I<is> significant in verbatim paragraphs (although, in +processing, tabs are probably expanded). + +=item * + +An B<ordinary paragraph>. A paragraph is an ordinary paragraph +if its first line matches neither C<m/\A=[a-zA-Z]/> nor +C<m/\A[ \t]/>, I<and> if it's not inside a "=begin I<identifier>", +... "=end I<identifier>" sequence unless "I<identifier>" begins with +a colon (":"). + +=item * + +A B<data paragraph>. This is a paragraph that I<is> inside a "=begin +I<identifier>" ... "=end I<identifier>" sequence where +"I<identifier>" does I<not> 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 I<around> it. + +=back + +For example: consider the following paragraphs: + + # <- that's the 0th column + + =head1 Foo + + Stuff + + $foo->bar + + =cut + +Here, "=head1 Foo" and "=cut" are command paragraphs because the first +line of each matches C<m/\A=[a-zA-Z]/>. "I<[space][space]>$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<identifier>" ... "=end I<identifier>" commands stop +paragraphs that they surround from being parsed as ordinary or verbatim +paragraphs, if I<identifier> doesn't begin with a colon. This +is discussed in detail in the section +L</About Data Paragraphs and "=beginE<sol>=end" Regions>. + +=head1 Pod Commands + +This section is intended to supplement and clarify the discussion in +L<perlpod/"Command Paragraph">. These are the currently recognized +Pod commands: + +=over + +=item "=head1", "=head2", "=head3", "=head4" + +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! + +=item "=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. + +=item "=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 I<start> 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. + +=item "=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 L</"About =over...=back Regions"> section, further +below. Formatting codes are not expanded. Examples: + + =over 3 + + =over 3.5 + + =over + +=item "=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 L</"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. + +=item "=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. + +=item "=begin formatname" + +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" I<does> begin +with a colon, then non-command paragraphs are ordinary paragraphs +or data paragraphs. This is discussed in detail in the section +L</About Data Paragraphs and "=beginE<sol>=end" Regions>. + +It is advised that formatnames match the regexp +C<m/\A:?[-a-zA-Z0-9_]+\z/>. Implementors should anticipate future +expansion in the semantics and syntax of the first parameter +to "=begin"/"=end"/"=for". + +=item "=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 +L</About Data Paragraphs and "=beginE<sol>=end" Regions>. + +=item "=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" I<doesn'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. + +=item "=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 I<encodingname>, which must be +an encoding name that L<Encode> recognizes. (Encode's list +of supported encodings, in L<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). + +=back + +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. + + + +=head1 Pod Formatting Codes + +(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: + +=over + +=item * + +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<dump()> for? + + X<C<chmod> and C<unlink()> Under Different Operating Systems> + +=item * + +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 "CE<lt><<" +and before the ">>" (or whatever letter) are I<not> 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. + +=back + +In parsing Pod, a notably tricky part is the correct parsing of +(potentially nested!) formatting codes. Implementors should +consult the code in the C<parse_text> routine in Pod::Parser as an +example of a correct implementation. + +=over + +=item C<IE<lt>textE<gt>> -- italic text + +See the brief discussion in L<perlpod/"Formatting Codes">. + +=item C<BE<lt>textE<gt>> -- bold text + +See the brief discussion in L<perlpod/"Formatting Codes">. + +=item C<CE<lt>codeE<gt>> -- code text + +See the brief discussion in L<perlpod/"Formatting Codes">. + +=item C<FE<lt>filenameE<gt>> -- style for filenames + +See the brief discussion in L<perlpod/"Formatting Codes">. + +=item C<XE<lt>topic nameE<gt>> -- an index entry + +See the brief discussion in L<perlpod/"Formatting Codes">. + +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. + +=item C<ZE<lt>E<gt>> -- a null (zero-effect) formatting code + +Discussed briefly in L<perlpod/"Formatting Codes">. + +This code is unusual is that it should have no content. That is, +a processor may complain if it sees C<ZE<lt>potatoesE<gt>>. Whether +or not it complains, the I<potatoes> text should ignored. + +=item C<LE<lt>nameE<gt>> -- a hyperlink + +The complicated syntaxes of this code are discussed at length in +L<perlpod/"Formatting Codes">, and implementation details are +discussed below, in L</"About LE<lt>...E<gt> Codes">. Parsing the +contents of LE<lt>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, +I<before> EE<lt>...> codes are resolved. + +=item C<EE<lt>escapeE<gt>> -- a character escape + +See L<perlpod/"Formatting Codes">, and several points in +L</Notes on Implementing Pod Processors>. + +=item C<SE<lt>textE<gt>> -- 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, "$z". The +difference is that in the latter, with the S code, those spaces +are not "normal" spaces, but instead are non-breaking spaces. + +=back + + +If a Pod processor sees any formatting code other than the ones +listed above (as in "NE<lt>...>", or "QE<lt>...>", etc.), that +processor must by default treat this as an error. +A Pod parser may allow a way for particular +applications to add to the above list of known formatting codes; +a Pod parser might even allow a way to stipulate, for each additional +command, whether it requires some form of special processing, as +LE<lt>...> does. + +Future versions of this specification may add additional +formatting codes. + +Historical note: A few older Pod processors would not see a ">" as +closing a "CE<lt>" code, if the ">" was immediately preceded by +a "-". This was so that this: + + C<$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 I<not> 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.) + + + +=head1 Notes on Implementing Pod Processors + +The following is a long section of miscellaneous requirements +and suggestions to do with Pod processing. + +=over + +=item * + +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. + +=item * + +Pod parsers must recognize I<all> of the three well-known newline +formats: CR, LF, and CRLF. See L<perlport|perlport>. + +=item * + +Pod parsers should accept input lines that are of any length. + +=item * + +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 Latin-1. + +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. + +=item * + +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. If the file begins with the three literal byte values +0xEF 0xBB 0xBF, this is the BOM for UTF-8. + +=for comment + use bytes; print map sprintf(" 0x%02X", ord $_), split '', "\x{feff}"; + 0xEF 0xBB 0xBF + +=for comment + If toke.c is modified to support UTF-32, add mention of those here. + +=item * + +A naive but sufficient heuristic 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 0xC0 - 0xFD +I<and> 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 Latin-1. 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 I<not> 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. + +=for comment + If/WHEN some brave soul makes these heuristics into a generic + text-file class (or PerlIO layer?), we can presumably delete + mention of these icky details from this file, and can instead + tell people to just use appropriate class/layer. + Auto-recognition of newline sequences would be another desirable + feature of such a class/layer. + HINT HINT HINT. + +=for comment + "The probability that a string of characters + in any other encoding appears as valid UTF-8 is low" - RFC2279 + +=item * + +This document's requirements and suggestions about encodings +do not apply to Pod processors running on non-ASCII platforms, +notably EBCDIC platforms. + +=item * + +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.) + +=item * + +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 C<die>ing). + +=item * + +Pod parsers I<may> emit warnings or error messages ("Unknown E code +EE<lt>zslig>!") to STDERR (whether through printing to STDERR, or +C<warn>ing/C<carp>ing, or C<die>ing/C<croak>ing), but I<must> 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. + +=item * + +In cases of exceptionally aberrant documents, Pod parsers may abort the +parse. Even then, using C<die>ing/C<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. + +=item * + +In paragraphs where formatting codes (like EE<lt>...>, BE<lt>...>) +are understood (i.e., I<not> verbatim paragraphs, but I<including> +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). + +=item * + +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 I<must never> do any of those things to text +in CE<lt>...> formatting codes, and never I<ever> to text in verbatim +paragraphs. + +=item * + +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. + +=item * + +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.) + +=item * + +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. + +=item * + +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. + +=item * + +Pod parsers, when reporting errors, should make some effort to report +an approximate line number ("Nested EE<lt>>'s in Paragraph #52, near +line 633 of Thing/Foo.pm!"), instead of merely noting the paragraph +number ("Nested EE<lt>>'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 EE<lt>>'s in +Paragraph #52 of Thing/Foo.pm, which begins 'Read/write accessor for +the CE<lt>interest rate> attribute...'"). + +=item * + +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. + +=item * + +Pod formatters, where feasible, are advised to avoid splitting short +verbatim paragraphs (under twelve lines, say) across pages. + +=item * + +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.) + +=item * + +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::Parser, comes with modern versions of Perl. + +=item * + +Characters in Pod documents may be conveyed either as literals, or by +number in EE<lt>n> codes, or by an equivalent mnemonic, as in +EE<lt>eacute> which is exactly equivalent to EE<lt>233>. + +Characters 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 +in the ranges 0-31 and 127-159 should not be used (neither as +literals, nor as EE<lt>number> codes), except for the +literal byte-sequences for newline (13, 13 10, or 10), and tab (9). + +Characters in the range 160-255 refer to Latin-1 characters (also +defined there by Unicode, with the same meaning). Characters above +255 should be understood to refer to Unicode characters. + +=item * + +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. + +=item * + +Besides the well-known "EE<lt>lt>" and "EE<lt>gt>" codes for +less-than and greater-than, Pod parsers must understand "EE<lt>sol>" +for "/" (solidus, slash), and "EE<lt>verbar>" for "|" (vertical bar, +pipe). Pod parsers should also understand "EE<lt>lchevron>" and +"EE<lt>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 "EE<lt>laquo>" +and "EE<lt>raquo>".) + +=item * + +Pod parsers should understand all "EE<lt>html>" codes as defined +in the entity declarations in the most recent XHTML specification at +C<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 "EE<lt>I<identifier>>" 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<identifier>, greater-than. Or Pod parsers may offer the +alternative option of processing such unknown +"EE<lt>I<identifier>>" codes by firing an event especially +for such codes, or by adding a special node-type to the in-memory +document tree. Such "EE<lt>I<identifier>>" may have special meaning +to some processors, or some processors may choose to add them to +a special error report. + +=item * + +Pod parsers must also support the XHTML codes "EE<lt>quot>" for +character 34 (doublequote, "), "EE<lt>amp>" for character 38 +(ampersand, &), and "EE<lt>apos>" for character 39 (apostrophe, '). + +=item * + +Note that in all cases of "EE<lt>whatever>", I<whatever> (whether +an htmlname, or a number in any base) must consist only of +alphanumeric characters -- that is, I<whatever> must watch +C<m/\A\w+\z/>. So "EE<lt> 0 1 2 3 >" is invalid, because +it contains spaces, which aren't alphanumeric characters. This +presumably does not I<need> 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 "EE<lt> 0 1 2 3 >" or "EE<lt>e-acute>" as I<syntactically> +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 "EE<lt>qacute>" +[sic]. However, Pod parsers are not required to make this +distinction. + +=item * + +Note that EE<lt>number> I<must not> be interpreted as simply +"codepoint I<number> in the current/native character set". It always +means only "the character represented by codepoint I<number> in +Unicode." (This is identical to the semantics of &#I<number>; 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 EE<lt>...> 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. + +=item * + +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 F<xhtml-symbol.ent>, 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 C<∞>, C<∞>, or C<∞>), reducing the need +for idiosyncratic mappings of Unicode-to-I<my_escapes>. + +=item * + +It is up to individual Pod formatter to display good judgement when +confronted with an unrenderable character (which is distinct from an +unknown EE<lt>thing> sequence that the parser couldn't resolve to +anything, renderable or not). It is good practice to map Latin letters +with diacritics (like "EE<lt>eacute>"/"EE<lt>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 EE<lt>233> to "e"), Pod formatters may use the +%Latin1Code_to_fallback table in L<Pod::Escapes|Pod::Escapes>, or +L<Text::Unidecode|Text::Unidecode>, if available. + +For example, this Pod text: + + magic is enabled if you set C<$Currency> to 'E<euro>'. + +may be rendered as: +"magic is enabled if you set C<$Currency> to 'I<?>'" or as +"magic is enabled if you set C<$Currency> to 'B<[euro]>'", or as +"magic is enabled if you set C<$Currency> to '[x20AC]', etc. + +A Pod formatter may also note, in a comment or warning, a list of what +unrenderable characters were encountered. + +=item * + +EE<lt>...> may freely appear in any formatting code (other than +in another EE<lt>...> or in an ZE<lt>>). That is, "XE<lt>The +EE<lt>euro>1,000,000 Solution>" is valid, as is "LE<lt>The +EE<lt>euro>1,000,000 Solution|Million::Euros>". + +=item * + +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 "EE<lt>160>" or +"EE<lt>nbsp>" code); and Pod can contain "SE<lt>foo +IE<lt>barE<gt> 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 "SE<lt>foo +IE<lt>barE<gt> baz>" as if it were +"fooI<NBSP>IE<lt>barE<gt>I<NBSP>baz", and, going the other way, the +optional parsing of groups of words joined by NBSP's as if each group +were in a SE<lt>...> code, so that formatters may use the +representation that maps best to what the output format demands. + +=item * + +Some processors may find that the C<SE<lt>...E<gt>> 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 I<not> to +spaces in I<all> text, but I<only> to spaces in I<printable> 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". + +=item * + +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. C<EE<lt>173E<gt>> = C<EE<lt>0xADE<gt>> = +C<EE<lt>shyE<gt>>). 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-I<[linebreak]>action" or "manu-I<[linebreak]>script" +(and if it doesn't hyphenate it, then the C<EE<lt>shyE<gt>> 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 C<EE<lt>shyE<gt>> code. + +In practice, it is anticipated that this character will not be used +often, but formatters should either support it, or delete it. + +=item * + +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". + +=item * + +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. + +=back + + + + + +=head1 About LE<lt>...E<gt> Codes + +As you can tell from a glance at L<perlpod|perlpod>, the LE<lt>...> +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. + +=over + +=item * + +In parsing an LE<lt>...> code, Pod parsers must distinguish at least +four attributes: + +=over + +=item First: + +The link-text. If there is none, this must be undef. (E.g., in +"LE<lt>Perl Functions|perlfunc>", the link-text is "Perl Functions". +In "LE<lt>Time::HiRes>" and even "LE<lt>|Time::HiRes>", there is no +link text. Note that link text may contain formatting.) + +=item 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 +"LE<lt>Getopt::Std>", the inferred link text is "Getopt::Std".) + +=item Third: + +The name or URL, or undef if none. (E.g., in "LE<lt>Perl +Functions|perlfunc>", the name -- also sometimes called the page -- +is "perlfunc". In "LE<lt>/CAVEATS>", the name is undef.) + +=item Fourth: + +The section (AKA "item" in older perlpods), or undef if none. E.g., +in "LE<lt>Getopt::Std/DESCRIPTIONE<gt>", "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".) + +=back + +Pod parsers may also note additional attributes including: + +=over + +=item 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 "crontab(5)" is). + +=item Sixth: + +The raw original LE<lt>...> content, before text is split on +"|", "/", etc, and before EE<lt>...> codes are expanded. + +=back + +(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" # orig. 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<http://www.perl.org/> + => undef, # link text + "http://www.perl.org/", # possibly inferred link text + "http://www.perl.org/", # name + undef, # section + 'url', # what sort of link + "http://www.perl.org/" # original content + +Note that you can distinguish URL-links from anything else by the +fact that they match C<m/\A\w+:[^:\s]\S*\z/>. So +C<LE<lt>http://www.perl.comE<gt>> is a URL, but +C<LE<lt>HTTP::ResponseE<gt>> isn't. + +=item * + +In case of LE<lt>...> codes with no "text|" part in them, +older formatters have exhibited great variation in actually displaying +the link or cross reference. For example, LE<lt>crontab(5)> would render +as "the C<crontab(5)> manpage", or "in the C<crontab(5)> manpage" +or just "C<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> + +=item * + +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> + +=item * + +Previous versions of perlpod distinguished C<LE<lt>name/"section"E<gt>> +links from C<LE<lt>name/itemE<gt>> links (and their targets). These +have been merged syntactically and semantically in the current +specification, and I<section> can refer either to a "=headI<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 I<section> identifier (e.g., in HTML, several things all producing +the same I<anchorname> in <a name="I<anchorname>">...</a> +elements). Where Pod processors can control this behavior, they should +use the first such anchor. That is, C<LE<lt>Foo/BarE<gt>> refers to the +I<first> "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="I<anchorname>">...</a> is most easily just left up to +browsers to decide. + +=item * + +Authors wanting to link to a particular (absolute) URL, must do so +only with "LE<lt>scheme:...>" codes (like +LE<lt>http://www.perl.org>), and must not attempt "LE<lt>Some Site +Name|scheme:...>" codes. This restriction avoids many problems +in parsing and rendering LE<lt>...> codes. + +=item * + +In a C<LE<lt>text|...E<gt>> code, text may contain formatting codes +for formatting or for EE<lt>...> escapes, as in: + + L<B<ummE<234>stuff>|...> + +For C<LE<lt>...E<gt>> codes without a "name|" part, only +C<EE<lt>...E<gt>> and C<ZE<lt>E<gt>> codes may occur -- no +other formatting codes. That is, authors should not use +"C<LE<lt>BE<lt>Foo::BarE<gt>E<gt>>". + +Note, however, that formatting codes and ZE<lt>>'s can occur in any +and all parts of an LE<lt>...> (i.e., in I<name>, I<section>, I<text>, +and I<url>). + +Authors must not nest LE<lt>...> codes. For example, "LE<lt>The +LE<lt>Foo::Bar> man page>" should be treated as an error. + +=item * + +Note that Pod authors may use formatting codes inside the "text" +part of "LE<lt>text|name>" (and so on for LE<lt>text|/"sec">). + +In other words, this is valid: + + Go read L<the docs on C<$.>|perlvar/"$."> + +Some output formats that do allow rendering "LE<lt>...>" codes as +hypertext, might not allow the link-text to be formatted; in +that case, formatters will have to just ignore that formatting. + +=item * + +At time of writing, C<LE<lt>nameE<gt>> values are of two types: +either the name of a Pod page like C<LE<lt>Foo::BarE<gt>> (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 C<LE<lt>crontab(5)E<gt>>. In theory, C<LE<lt>chmodE<gt>> +in 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 "crontab(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 C<LE<lt>fooE<gt>> code. + +=item * + +Previous versions of perlpod allowed for a C<LE<lt>sectionE<gt>> syntax +(as in C<LE<lt>Object AttributesE<gt>>), which was not easily distinguishable +from C<LE<lt>nameE<gt>> syntax. This syntax is no longer in the +specification, and has been replaced by the C<LE<lt>"section"E<gt>> syntax +(where the quotes were formerly optional). Pod parsers should tolerate +the C<LE<lt>sectionE<gt>> syntax, for a while at least. The suggested +heuristic for distinguishing C<LE<lt>sectionE<gt>> from C<LE<lt>nameE<gt>> +is that if it contains any whitespace, it's a I<section>. Pod processors +may warn about this being deprecated syntax. + +=back + +=head1 About =over...=back Regions + +"=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".) + +=over + +=item * + +The non-zero numeric I<indentlevel> in "=over I<indentlevel>" ... +"=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<indentlevel> parameter is +equivalent to an I<indentlevel> value of 4. Pod processors may +complain if I<indentlevel> is present but is not a positive number +matching C<m/\A(\d*\.)?\d+\z/>. + +=item * + +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>. + +=item * + +Each "=over" ... "=back" region should be one of the following: + +=over + +=item * + +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. + +=item * + +An "=over" ... "=back" region containing only +C<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.) + +=item * + +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 +C<m/\A=item\s+\d+\.?\s*\z/> or C<m/\A=item\s+\*\s*\z/>, nor should it +match just C<m/\A=item\s*\z/>. + +=item * + +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. + +=back + +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. + +=item * + +Pod formatters I<must> tolerate arbitrarily large amounts of text +in the "=item I<text...>" 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. + +=item * + +Pod processors should tolerate "=item *" / "=item I<number>" 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 + +=item * + +No "=over" ... "=back" region can contain headings. Processors may +treat such a heading as an error. + +=item * + +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. + +=item * + +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. + +=item * + +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. + +=back + + + +=head1 About Data Paragraphs and "=begin/=end" Regions + +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 "EE<lt>/em>" (in the first paragraph) as a formatting +code, just like "EE<lt>lt>" or "EE<lt>eacute>". But since this +is in a "=begin I<identifier>"..."=end I<identifier>" region I<and> +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 +I<lack> of a ":" prefix.) + +Note that a "=begin I<identifier>"..."=end I<identifier>" region where +I<identifier> begins with a colon, I<can> 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<identifier>"..."=end I<identifier>" +region where I<identifier> does I<not> 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 +I<not> 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 I<aren'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 I<only> 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") I<doesn'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" I<may> be stored +as two data paragraphs (one consisting of +"<img src='wirth_spokesmodeling_book.png'>\n" +and another consisting of "<hr>\n"), but I<should> 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 I<something>"..."=end I<something>" regions, +empty "=begin :I<something>"..."=end :I<something>" regions, and +contentless "=for I<something>" and "=for :I<something>" +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" -- i.e., 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. + +=head1 SEE ALSO + +L<perlpod>, L<perlsyn/"PODs: Embedded Documentation">, +L<podchecker> + +=head1 AUTHOR + +Sean M. Burke + +=cut + + |