NAME

Pod::Parser - base class for creating POD filters and translators

SYNOPSIS

use Pod::Parser;

package MyParser;
@ISA = qw(Pod::Parser);

sub command { 
    my ($parser, $command, $paragraph, $line_num) = @_;
    ## Interpret the command and its text; sample actions might be:
    if ($command eq 'head1') { ... }
    elsif ($command eq 'head2') { ... }
    ## ... other commands and their actions
    my $out_fh = $parser->output_handle();
    my $expansion = $parser->interpolate($paragraph, $line_num);
    print $out_fh $expansion;
}

sub verbatim { 
    my ($parser, $paragraph, $line_num) = @_;
    ## Format verbatim paragraph; sample actions might be:
    my $out_fh = $parser->output_handle();
    print $out_fh $paragraph;
}

sub textblock { 
    my ($parser, $paragraph, $line_num) = @_;
    ## Translate/Format this block of text; sample actions might be:
    my $out_fh = $parser->output_handle();
    my $expansion = $parser->interpolate($paragraph, $line_num);
    print $out_fh $expansion;
}

sub interior_sequence { 
    my ($parser, $seq_command, $seq_argument) = @_;
    ## Expand an interior sequence; sample actions might be:
    return "*$seq_argument*"     if ($seq_command = 'B');
    return "`$seq_argument'"     if ($seq_command = 'C');
    return "_${seq_argument}_'"  if ($seq_command = 'I');
    ## ... other sequence commands and their resulting text
}

package main;

## Create a parser object and have it parse file whose name was
## given on the command-line (use STDIN if no files were given).
$parser = new MyParser();
$parser->parse_from_filehandle(\*STDIN)  if (@ARGV == 0);
for (@ARGV) { $parser->parse_from_file($_); }

REQUIRES

perl5.004, Pod::InputObjects, Exporter, FileHandle, Carp

EXPORTS

Nothing.

DESCRIPTION

Pod::Parser is a base class for creating POD filters and translators. It handles most of the effort involved with parsing the POD sections from an input stream, leaving subclasses free to be concerned only with performing the actual translation of text.

Pod::Parser parses PODs, and makes method calls to handle the various components of the POD. Subclasses of Pod::Parser override these methods to translate the POD into whatever output format they desire.

QUICK OVERVIEW

To create a POD filter for translating POD documentation into some other format, you create a subclass of Pod::Parser which typically overrides just the base class implementation for the following methods:

command()
verbatim()
textblock()
interior_sequence()

You may also want to override the begin_input() and end_input() methods for your subclass (to perform any needed per-file and/or per-document initialization or cleanup).

If you need to perform any preprocesssing of input before it is parsed you may want to override one or more of preprocess_line() and/or preprocess_paragraph().

Sometimes it may be necessary to make more than one pass over the input files. If this is the case you have several options. You can make the first pass using Pod::Parser and override your methods to store the intermediate results in memory somewhere for the end_pod() method to process. You could use Pod::Parser for several passes with an appropriate state variable to control the operation for each pass. If your input source can't be reset to start at the beginning, you can store it in some other structure as a string or an array and have that structure implement a getline() method (which is all that parse_from_filehandle() uses to read input).

Feel free to add any member data fields you need to keep track of things like current font, indentation, horizontal or vertical position, or whatever else you like. Be sure to read "PRIVATE METHODS AND DATA" to avoid name collisions.

For the most part, the Pod::Parser base class should be able to do most of the input parsing for you and leave you free to worry about how to intepret the commands and translate the result.

RECOMMENDED SUBROUTINE/METHOD OVERRIDES

Pod::Parser provides several methods which most subclasses will probably want to override. These methods are as follows:

command()

$parser->command($cmd,$text,$line_num,$pod_para);

This method should be overridden by subclasses to take the appropriate action when a POD command paragraph (denoted by a line beginning with "=") is encountered. When such a POD directive is seen in the input, this method is called and is passed:

$cmd: the name of the command for this POD paragraph
$text: the paragraph text for the given POD paragraph command.
$line_num: the line-number of the beginning of the paragraph
$pod_para: a reference to a Pod::Paragraph object which contains further information about the paragraph command (see Pod::InputObjects for details).

Note that this method is called for =pod paragraphs.

The base class implementation of this method simply treats the raw POD command as normal block of paragraph text (invoking the textblock() method with the command paragraph).

verbatim()

$parser->verbatim($text,$line_num,$pod_para);

This method may be overridden by subclasses to take the appropriate action when a block of verbatim text is encountered. It is passed the following parameters:

$text: the block of text for the verbatim paragraph
$line_num: the line-number of the beginning of the paragraph
$pod_para: a reference to a Pod::Paragraph object which contains further information about the paragraph (see Pod::InputObjects for details).

The base class implementation of this method simply prints the textblock (unmodified) to the output filehandle.

textblock()

$parser->textblock($text,$line_num,$pod_para);

This method may be overridden by subclasses to take the appropriate action when a normal block of POD text is encountered (although the base class method will usually do what you want). It is passed the following parameters:

$text: the block of text for the a POD paragraph
$line_num: the line-number of the beginning of the paragraph
$pod_para: a reference to a Pod::Paragraph object which contains further information about the paragraph (see Pod::InputObjects for details).

In order to process interior sequences, subclasses implementations of this method will probably want to invoke either interpolate() or parse_text(), passing it the text block $text, and the corresponding line number in $line_num, and then perform any desired processing upon the returned result.

The base class implementation of this method simply prints the text block as it occurred in the input stream).

interior_sequence()

$parser->interior_sequence($seq_cmd,$seq_arg,$pod_seq);

This method should be overridden by subclasses to take the appropriate action when an interior sequence is encountered. An interior sequence is an embedded command within a block of text which appears as a command name (usually a single uppercase character) followed immediately by a string of text which is enclosed in angle brackets. This method is passed the sequence command $seq_cmd and the corresponding text $seq_arg. It is invoked by the interpolate() method for each interior sequence that occurs in the string that it is passed. It should return the desired text string to be used in place of the interior sequence. The $pod_seq argument is a reference to a Pod::InteriorSequence object which contains further information about the interior sequence. Please see Pod::InputObjects for details if you need to access this additional information.

Subclass implementations of this method may wish to invoke the nested() method of $pod_seq to see if it is nested inside some other interior-sequence (and if so, which kind).

The base class implementation of the interior_sequence() method simply returns the raw text of the interior sequence (as it occurred in the input) to the caller.

OPTIONAL SUBROUTINE/METHOD OVERRIDES

Pod::Parser provides several methods which subclasses may want to override to perform any special pre/post-processing. These methods do not have to be overridden, but it may be useful for subclasses to take advantage of them.

new()

my $parser = Pod::Parser->new();

This is the constructor for Pod::Parser and its subclasses. You do not need to override this method! It is capable of constructing subclass objects as well as base class objects, provided you use any of the following constructor invocation styles:

my $parser1 = MyParser->new();
my $parser2 = new MyParser();
my $parser3 = $parser2->new();

where MyParser is some subclass of Pod::Parser.

Using the syntax MyParser::new() to invoke the constructor is not recommended, but if you insist on being able to do this, then the subclass will need to override the new() constructor method. If you do override the constructor, you must be sure to invoke the initialize() method of the newly blessed object.

Using any of the above invocations, the first argument to the constructor is always the corresponding package name (or object reference). No other arguments are required, but if desired, an associative array (or hash-table) my be passed to the new() constructor, as in:

my $parser1 = MyParser->new( MYDATA => $value1, MOREDATA => $value2 );
my $parser2 = new MyParser( -myflag => 1 );

All arguments passed to the new() constructor will be treated as key/value pairs in a hash-table. The newly constructed object will be initialized by copying the contents of the given hash-table (which may have been empty). The new() constructor for this class and all of its subclasses returns a blessed reference to the initialized object (hash-table).

initialize()

$parser->initialize();

This method performs any necessary object initialization. It takes no arguments (other than the object instance of course, which is typically copied to a local variable named $self). If subclasses override this method then they must be sure to invoke $self->SUPER::initialize().

begin_pod()

$parser->begin_pod();

This method is invoked at the beginning of processing for each POD document that is encountered in the input. Subclasses should override this method to perform any per-document initialization.

begin_input()

$parser->begin_input();

This method is invoked by parse_from_filehandle() immediately before processing input from a filehandle. The base class implementation does nothing, however, subclasses may override it to perform any per-file initializations.

Note that if multiple files are parsed for a single POD document (perhaps the result of some future =include directive) this method is invoked for every file that is parsed. If you wish to perform certain initializations once per document, then you should use begin_pod().

end_input()

$parser->end_input();

This method is invoked by parse_from_filehandle() immediately after processing input from a filehandle. The base class implementation does nothing, however, subclasses may override it to perform any per-file cleanup actions.

Please note that if multiple files are parsed for a single POD document (perhaps the result of some kind of =include directive) this method is invoked for every file that is parsed. If you wish to perform certain cleanup actions once per document, then you should use end_pod().

end_pod()

$parser->end_pod();

This method is invoked at the end of processing for each POD document that is encountered in the input. Subclasses should override this method to perform any per-document finalization.

preprocess_line()

$textline = $parser->preprocess_line($text, $line_num);

This method should be overridden by subclasses that wish to perform any kind of preprocessing for each line of input (before it has been determined whether or not it is part of a POD paragraph). The parameter $text is the input line; and the parameter $line_num is the line number of the corresponding text line.

The value returned should correspond to the new text to use in its place. If the empty string or an undefined value is returned then no further processing will be performed for this line.

Please note that the preprocess_line() method is invoked before the preprocess_paragraph() method. After all (possibly preprocessed) lines in a paragraph have been assembled together and it has been determined that the paragraph is part of the POD documentation from one of the selected sections, then preprocess_paragraph() is invoked.

The base class implementation of this method returns the given text.

preprocess_paragraph()

$textblock = $parser->preprocess_paragraph($text, $line_num);

This method should be overridden by subclasses that wish to perform any kind of preprocessing for each block (paragraph) of POD documentation that appears in the input stream. The parameter $text is the POD paragraph from the input file; and the parameter $line_num is the line number for the beginning of the corresponding paragraph.

The value returned should correspond to the new text to use in its place If the empty string is returned or an undefined value is returned, then the given $text is ignored (not processed).

This method is invoked after gathering up all thelines in a paragraph but before trying to further parse or interpret them. After preprocess_paragraph() returns, the current cutting state (which is returned by $self->cutting()) is examined. If it evaluates to false then input text (including the given $text) is cut (not processed) until the next POD directive is encountered.

The base class implementation of this method returns the given text.

METHODS FOR PARSING AND PROCESSING

Pod::Parser provides several methods to process input text. These methods typically won't need to be overridden, but subclasses may want to invoke them to exploit their functionality.

parse_text()

$ptree1 = $parser->parse_text($text, $line_num);
$ptree2 = $parser->parse_text({%opts}, $text, $line_num);
$ptree3 = $parser->parse_text(\%opts, $text, $line_num);

This method is useful if you need to perform your own interpolation of interior sequences and can't rely upon interpolate to expand them in simple bottom-up order order.

The parameter $text is a string or block of text to be parsed for interior sequences; and the parameter $line_num is the line number curresponding to the beginning of $text.

parse_text() will parse the given text into a parse-tree of "nodes." and interior-sequences. Each "node" in the parse tree is either a text-string, or a Pod::InteriorSequence. The result returned is a parse-tree of type Pod::ParseTree. Please see Pod::InputObjects for more information about Pod::InteriorSequence and Pod::ParseTree.

If desired, an optional hash-ref may be specified as the first argument to customize certain aspects of the parse-tree that is created and returned. The set of recognized option keywords are:

-expand_seq => code-ref|method-name

Normally, the parse-tree returned by parse_text() will contain an unexpanded Pod::InteriorSequence object for each interior-sequence encountered. Specifying -expand_seq tells parse_text() to "expand" every interior-sequence it sees by invoking the referenced function (or named method of the parser object) and using the return value as the expanded result.

If a subroutine reference was given, it is invoked as:

&$code_ref( $parser, $sequence )

and if a method-name was given, it is invoked as:

$parser->method_name( $sequence )

where $parser is a reference to the parser object, and $sequence is a reference to the interior-sequence object. [NOTE: If the interior_sequence() method is specified, then it is invoked according to the interface specified in "interior_sequence()"].

-expand_ptree => code-ref|method-name

Rather than returning a Pod::ParseTree, pass the parse-tree as an argument to the referenced subroutine (or named method of the parser object) and return the result instead of the parse-tree object.

If a subroutine reference was given, it is invoked as:

&$code_ref( $parser, $ptree )

and if a method-name was given, it is invoked as:

$parser->method_name( $ptree )

where $parser is a reference to the parser object, and $ptree is a reference to the parse-tree object.

interpolate()

$textblock = $parser->interpolate($text, $line_num);

This method translates all text (including any embedded interior sequences) in the given text string $text and returns the interpolated result. The parameter $line_num is the line number corresponding to the beginning of $text.

interpolate() merely invokes a private method to recursively expand nested interior sequences in bottom-up order (innermost sequences are expanded first). If there is a need to expand nested sequences in some alternate order, use parse_text instead.

parse_from_filehandle()

$parser->parse_from_filehandle($in_fh,$out_fh);

This method takes an input filehandle (which is assumed to already be opened for reading) and reads the entire input stream looking for blocks (paragraphs) of POD documentation to be processed. If no first argument is given the default input filehandle STDIN is used.

The $in_fh parameter may be any object that provides a getline() method to retrieve a single line of input text (hence, an appropriate wrapper object could be used to parse PODs from a single string or an array of strings).

Using $in_fh->getline(), input is read line-by-line and assembled into paragraphs or "blocks" (which are separated by lines containing nothing but whitespace). For each block of POD documentation encountered it will invoke a method to parse the given paragraph.

If a second argument is given then it should correspond to a filehandle where output should be sent (otherwise the default output filehandle is STDOUT if no output filehandle is currently in use).

NOTE: For performance reasons, this method caches the input stream at the top of the stack in a local variable. Any attempts by clients to change the stack contents during processing when in the midst executing of this method will not affect the input stream used by the current invocation of this method.

This method does not usually need to be overridden by subclasses.

parse_from_file()

$parser->parse_from_file($filename,$outfile);

This method takes a filename and does the following:

opens the input and output files for reading (creating the appropriate filehandles)
invokes the parse_from_filehandle() method passing it the corresponding input and output filehandles.
closes the input and output files.

If the special input filename "-" or "<&STDIN" is given then the STDIN filehandle is used for input (and no open or close is performed). If no input filename is specified then "-" is implied.

If a second argument is given then it should be the name of the desired output file. If the special output filename "-" or ">&STDOUT" is given then the STDOUT filehandle is used for output (and no open or close is performed). If the special output filename ">&STDERR" is given then the STDERR filehandle is used for output (and no open or close is performed). If no output filehandle is currently in use and no output filename is specified, then "-" is implied.

This method does not usually need to be overridden by subclasses.

ACCESSOR METHODS

Clients of Pod::Parser should use the following methods to access instance data fields:

errorsub()

$parser->errorsub("method_name");
$parser->errorsub(\&warn_user);
$parser->errorsub(sub { print STDERR, @_ });

Specifies the method or subroutine to use when printing error messages about POD syntax. The supplied method/subroutine must return TRUE upon successful printing of the message. If undef is given, then the warn builtin is used to issue error messages (this is the default behavior).

my $errorsub = $parser->errorsub()
my $errmsg = "This is an error message!\n"
(ref $errorsub) and &{$errorsub}($errmsg)
    or (defined $errmsg) and $parser->$errorsub($errmsg)
        or  warn($errmsg);

Returns a method name, or else a reference to the user-supplied subroutine used to print error messages. Returns undef if the warn builtin is used to issue error messages (this is the default behavior).

cutting()

$boolean = $parser->cutting();

Returns the current cutting state: a boolean-valued scalar which evaluates to true if text from the input file is currently being "cut" (meaning it is not considered part of the POD document).

$parser->cutting($boolean);

Sets the current cutting state to the given value and returns the result.

output_file()

$fname = $parser->output_file();

Returns the name of the output file being written.

output_handle()

$fhandle = $parser->output_handle();

Returns the output filehandle object.

input_file()

$fname = $parser->input_file();

Returns the name of the input file being read.

input_handle()

$fhandle = $parser->input_handle();

Returns the current input filehandle object.

PRIVATE METHODS AND DATA

Pod::Parser makes use of several internal methods and data fields which clients should not need to see or use. For the sake of avoiding name collisions for client data and methods, these methods and fields are briefly discussed here. Determined hackers may obtain further information about them by reading the Pod::Parser source code.

Private data fields are stored in the hash-object whose reference is returned by the new() constructor for this class. The names of all private methods and data-fields used by Pod::Parser begin with a prefix of "_" and match the regular expression /^_\w+$/.

AUTHOR

Brad Appleton <bradapp@enteract.com>

Based on code for Pod::Text written by Tom Christiansen <tchrist@mox.perl.com>

To install Pod::Usage, copy and paste the appropriate command in to your terminal.

cpanm

cpanm Pod::Usage

CPAN shell

perl -MCPAN -e shell
install Pod::Usage

For more information on module installation, please visit the detailed CPAN module installation guide.

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