Provided by: libparser-mgc-perl_0.16-1_all bug

NAME

       "Parser::MGC" - build simple recursive-descent parsers

SYNOPSIS

        package My::Grammar::Parser
        use base qw( Parser::MGC );

        sub parse
        {
           my $self = shift;

           $self->sequence_of( sub {
              $self->any_of(
                 sub { $self->token_int },
                 sub { $self->token_string },
                 sub { \$self->token_ident },
                 sub { $self->scope_of( "(", \&parse, ")" ) }
              );
           } );
        }

        my $parser = My::Grammar::Parser->new;

        my $tree = $parser->from_file( $ARGV[0] );

        ...

DESCRIPTION

       This base class provides a low-level framework for building recursive-descent parsers that
       consume a given input string from left to right, returning a parse structure. It takes its
       name from the "m//gc" regexps used to implement the token parsing behaviour.

       It provides a number of token-parsing methods, which each extract a grammatical token from
       the string. It also provides wrapping methods that can be used to build up a possibly-
       recursive grammar structure, by applying a structure around other parts of parsing code.

   Backtracking
       Each method, both token and structural, atomically either consumes a prefix of the string
       and returns its result, or fails and consumes nothing. This makes it simple to implement
       grammars that require backtracking.

       Several structure-forming methods have some form of "optional" behaviour; they can
       optionally consume some amount of input or take some particular choice, but if the code
       invoked inside that subsequently fails, the structure can backtrack and take some
       different behaviour. This is usually what is required when testing whether the structure
       of the input string matches some part of the grammar that is optional, or has multiple
       choices.

       However, once the choice of grammar has been made, it is often useful to be able to fix on
       that one choice, thus making subsequent failures propagate up rather than taking that
       alternative behaviour. Control of this backtracking is given by the "commit" method; and
       careful use of this method is one of the key advantages that "Parser::MGC" has over more
       simple parsing using single regexps alone.

CONSTRUCTOR

   new
          $parser = Parser::MGC->new( %args )

       Returns a new instance of a "Parser::MGC" object. This must be called on a subclass that
       provides method of the name provided as "toplevel", by default called "parse".

       Takes the following named arguments

       toplevel => STRING
               Name of the toplevel method to use to start the parse from. If not supplied, will
               try to use a method called "parse".

       patterns => HASH
               Keys in this hash should map to quoted regexp ("qr//") references, to override the
               default patterns used to match tokens. See "PATTERNS" below

       accept_0o_oct => BOOL
               If true, the "token_int" method will also accept integers with a "0o" prefix as
               octal.

PATTERNS

       The following pattern names are recognised. They may be passed to the constructor in the
       "patterns" hash, or provided as a class method under the name "pattern_name".

       ·   ws

           Pattern used to skip whitespace between tokens. Defaults to "/[\s\n\t]+/"

       ·   comment

           Pattern used to skip comments between tokens. Undefined by default.

       ·   int

           Pattern used to parse an integer by "token_int". Defaults to
           "/-?(?:0x[[:xdigit:]]+|[[:digit:]]+)/". If "accept_0o_oct" is given, then this will be
           expanded to match "/0o[0-7]+/" as well.

       ·   float

           Pattern used to parse a floating-point number by "token_float". Defaults to
           "/-?(?:\d*\.\d+|\d+\.)(?:e-?\d+)?|-?\d+e-?\d+/i".

       ·   ident

           Pattern used to parse an identifier by "token_ident". Defaults to "/[[:alpha:]_]\w*/"

       ·   string_delim

           Pattern used to delimit a string by "token_string". Defaults to "/["']/".

METHODS

   from_string
          $result = $parser->from_string( $str )

       Parse the given literal string and return the result from the toplevel method.

   from_file
          $result = $parser->from_file( $file, %opts )

       Parse the given file, which may be a pathname in a string, or an opened IO handle, and
       return the result from the toplevel method.

       The following options are recognised:

       binmode => STRING
               If set, applies the given binmode to the filehandle before reading. Typically this
               can be used to set the encoding of the file.

                $parser->from_file( $file, binmode => ":encoding(UTF-8)" )

   from_reader
          $result = $parser->from_reader( \&reader )

       Since version 0.05.

       Parse the input which is read by the "reader" function. This function will be called in
       scalar context to generate portions of string to parse, being passed the $parser object.
       The function should return "undef" when it has no more string to return.

        $reader->( $parser )

       Note that because it is not generally possible to detect exactly when more input may be
       required due to failed regexp parsing, the reader function is only invoked during
       searching for skippable whitespace. This makes it suitable for reading lines of a file in
       the common case where lines are considered as skippable whitespace, or for reading lines
       of input interractively from a user. It cannot be used in all cases (for example, reading
       fixed-size buffers from a file) because two successive invocations may split a single
       token across the buffer boundaries, and cause parse failures.

   pos
          $pos = $parser->pos

       Since version 0.09.

       Returns the current parse position, as a character offset from the beginning of the file
       or string.

   take
          $str = $parser->take( $len )

       Since version 0.16.

       Returns the next $len characters directly from the input, prior to any whitespace or
       comment skipping. This does not take account of any pending end-of-scope marker that may
       be pending. It is intended for use by parsers of partially-binary protocols, or other
       situations in which it would be incorrect for the end-of-scope marker to take effect at
       this time.

   where
          ( $lineno, $col, $text ) = $parser->where

       Returns the current parse position, as a line and column number, and the entire current
       line of text. The first line is numbered 1, and the first column is numbered 0.

   fail
   fail_from
          $parser->fail( $message )

          $parser->fail_from( $pos, $message )

       "fail_from" since version 0.09.

       Aborts the current parse attempt with the given message string. The failure message will
       include the line and column position, and the line of input that failed at the current
       parse position ("fail"), or a position earlier obtained using the "pos" method
       ("fail_from").

       This failure will propagate up to the inner-most structure parsing method that has not
       been committed; or will cause the entire parser to fail if there are no further options to
       take.

   at_eos
          $eos = $parser->at_eos

       Returns true if the input string is at the end of the string.

   scope_level
          $level = $parser->scope_level

       Since version 0.05.

       Returns the number of nested "scope_of" calls that have been made.

STRUCTURE-FORMING METHODS

       The following methods may be used to build a grammatical structure out of the defined
       basic token-parsing methods. Each takes at least one code reference, which will be passed
       the actual $parser object as its first argument.

   maybe
          $ret = $parser->maybe( $code )

       Attempts to execute the given $code in scalar context, and returns what it returned,
       accepting that it might fail. $code may either be a CODE reference or a method name given
       as a string.

       If the code fails (either by calling "fail" itself, or by propagating a failure from
       another method it invoked) before it has invoked "commit", then none of the input string
       will be consumed; the current parsing position will be restored. "undef" will be returned
       in this case.

       If it calls "commit" then any subsequent failure will be propagated to the caller, rather
       than returning "undef".

       This may be considered to be similar to the "?" regexp qualifier.

        sub parse_declaration
        {
           my $self = shift;

           [ $self->parse_type,
             $self->token_ident,
             $self->maybe( sub {
                $self->expect( "=" );
                $self->parse_expression
             } ),
           ];
        }

   scope_of
          $ret = $parser->scope_of( $start, $code, $stop )

       Expects to find the $start pattern, then attempts to execute the given $code, then expects
       to find the $stop pattern. Returns whatever the code returned. $code may either be a CODE
       reference of a method name given as a string.

       While the code is being executed, the $stop pattern will be used by the token parsing
       methods as an end-of-scope marker; causing them to raise a failure if called at the end of
       a scope.

        sub parse_block
        {
           my $self = shift;

           $self->scope_of( "{", sub { $self->parse_statements }, "}" );
        }

       If the $start pattern is undefined, it is presumed the caller has already checked for
       this. This is useful when the stop pattern needs to be calculated based on the start
       pattern.

        sub parse_bracketed
        {
           my $self = shift;

           my $delim = $self->expect( qr/[\(\[\<\{]/ );
           $delim =~ tr/([<{/)]>}/;

           $self->scope_of( undef, sub { $self->parse_body }, $delim );
        }

       This method does not have any optional parts to it; any failures are immediately
       propagated to the caller.

   committed_scope_of
          $ret = $parser->committed_scope_of( $start, $code, $stop )

       Since version 0.16.

       A variant of "scope_of" that calls "commit" after a successful match of the start pattern.
       This is usually what you want if using "scope_of" from within an "any_of" choice, if no
       other alternative following this one could possibly match if the start pattern has.

   list_of
          $ret = $parser->list_of( $sep, $code )

       Expects to find a list of instances of something parsed by $code, separated by the $sep
       pattern. Returns an ARRAY ref containing a list of the return values from the $code. A
       single trailing delimiter is allowed, and does not affect the return value. $code may
       either be a CODE reference or a method name given as a string.

       This method does not consider it an error if the returned list is empty; that is, that the
       scope ended before any item instances were parsed from it.

        sub parse_numbers
        {
           my $self = shift;

           $self->list_of( ",", sub { $self->token_int } );
        }

       If the code fails (either by invoking "fail" itself, or by propagating a failure from
       another method it invoked) before it has invoked "commit" on a particular item, then the
       item is aborted and the parsing position will be restored to the beginning of that failed
       item. The list of results from previous successful attempts will be returned.

       If it calls "commit" within an item then any subsequent failure for that item will cause
       the entire "list_of" to fail, propagating that to the caller.

   sequence_of
          $ret = $parser->sequence_of( $code )

       A shortcut for calling "list_of" with an empty string as separator; expects to find at
       least one instance of something parsed by $code, separated only by skipped whitespace.

       This may be considered to be similar to the "+" or "*" regexp qualifiers.

        sub parse_statements
        {
           my $self = shift;

           $self->sequence_of( sub { $self->parse_statement } );
        }

       The interaction of failures in the code and the "commit" method is identical to that of
       "list_of".

   any_of
          $ret = $parser->any_of( @codes )

       Since version 0.06.

       Expects that one of the given code instances can parse something from the input, returning
       what it returned. Each code instance may indicate a failure to parse by calling the "fail"
       method or otherwise propagating a failure.  Each code instance may either be a CODE
       reference or a method name given as a string.

       This may be considered to be similar to the "|" regexp operator for forming alternations
       of possible parse trees.

        sub parse_statement
        {
           my $self = shift;

           $self->any_of(
              sub { $self->parse_declaration; $self->expect(";") },
              sub { $self->parse_expression; $self->expect(";") },
              sub { $self->parse_block },
           );
        }

       If the code for a given choice fails (either by invoking "fail" itself, or by propagating
       a failure from another method it invoked) before it has invoked "commit" itself, then the
       parsing position restored and the next choice will be attempted.

       If it calls "commit" then any subsequent failure for that choice will cause the entire
       "any_of" to fail, propagating that to the caller and no further choices will be attmepted.

   commit
          $parser->commit

       Calling this method will cancel the backtracking behaviour of the innermost "maybe",
       "list_of", "sequence_of", or "any_of" structure forming method.  That is, if later code
       then calls "fail", the exception will be propagated out of "maybe", no further list items
       will be attempted by "list_of" or "sequence_of", and no further code blocks will be
       attempted by "any_of".

       Typically this will be called once the grammatical structure alter has been determined,
       ensuring that any further failures are raised as real exceptions, rather than by
       attempting other alternatives.

        sub parse_statement
        {
           my $self = shift;

           $self->any_of(
              ...
              sub {
                 $self->scope_of( "{",
                    sub { $self->commit; $self->parse_statements; },
                 "}" ),
              },
           );
        }

       Though in this common pattern, "committed_scope_of" may be used instead.

TOKEN PARSING METHODS

       The following methods attempt to consume some part of the input string, to be used as part
       of the parsing process.

   expect
          $str = $parser->expect( $literal )

          $str = $parser->expect( qr/pattern/ )

          @groups = $parser->expect( qr/pattern/ )

       Expects to find a literal string or regexp pattern match, and consumes it.  In scalar
       context, this method returns the string that was captured. In list context it returns the
       matching substring and the contents of any subgroups contained in the pattern.

       This method will raise a parse error (by calling "fail") if the regexp fails to match.
       Note that if the pattern could match an empty string (such as for example "qr/\d*/"), the
       pattern will always match, even if it has to match an empty string. This method will not
       consider a failure if the regexp matches with zero-width.

   maybe_expect
          $str = $parser->maybe_expect( ... )

          @groups = $parser->maybe_expect( ... )

       Since version 0.10.

       A convenient shortcut equivalent to calling "expect" within "maybe", but implemented more
       efficiently, avoiding the exception-handling set up by "maybe". Returns "undef" or an
       empty list if the match fails.

   substring_before
          $str = $parser->substring_before( $literal )

          $str = $parser->substring_before( qr/pattern/ )

       Since version 0.06.

       Expects to possibly find a literal string or regexp pattern match. If it finds such,
       consume all the input text before but excluding this match, and return it. If it fails to
       find a match before the end of the current scope, consumes all the input text until the
       end of scope and return it.

       This method does not consume the part of input that matches, only the text before it. It
       is not considered a failure if the substring before this match is empty. If a non-empty
       match is required, use the "fail" method:

        sub token_nonempty_part
        {
           my $self = shift;

           my $str = $parser->substring_before( "," );
           length $str or $self->fail( "Expected a string fragment before ," );

           return $str;
        }

       Note that unlike most of the other token parsing methods, this method does not consume
       either leading or trailing whitespace around the substring. It is expected that this
       method would be used as part a parser to read quoted strings, or similar cases where
       whitespace should be preserved.

   generic_token
          $val = $parser->generic_token( $name, $re, $convert )

       Since version 0.08.

       Expects to find a token matching the precompiled regexp $re. If provided, the $convert
       CODE reference can be used to convert the string into a more convenient form. $name is
       used in the failure message if the pattern fails to match.

       If provided, the $convert function will be passed the parser and the matching substring;
       the value it returns is returned from "generic_token".

        $convert->( $parser, $substr )

       If not provided, the substring will be returned as it stands.

       This method is mostly provided for subclasses to define their own token types.  For
       example:

        sub token_hex
        {
           my $self = shift;
           $self->generic_token( hex => qr/[0-9A-F]{2}h/, sub { hex $_[1] } );
        }

   token_int
          $int = $parser->token_int

       Expects to find an integer in decimal, octal or hexadecimal notation, and consumes it.
       Negative integers, preceeded by "-", are also recognised.

   token_float
          $float = $parser->token_float

       Since version 0.04.

       Expects to find a number expressed in floating-point notation; a sequence of digits
       possibly prefixed by "-", possibly containing a decimal point, possibly followed by an
       exponent specified by "e" followed by an integer. The numerical value is then returned.

   token_number
          $number = $parser->token_number

       Since version 0.09.

       Expects to find a number expressed in either of the above forms.

   token_string
          $str = $parser->token_string

       Expects to find a quoted string, and consumes it. The string should be quoted using """ or
       "'" quote marks.

       The content of the quoted string can contain character escapes similar to those accepted
       by C or Perl. Specifically, the following forms are recognised:

        \a               Bell ("alert")
        \b               Backspace
        \e               Escape
        \f               Form feed
        \n               Newline
        \r               Return
        \t               Horizontal Tab
        \0, \012         Octal character
        \x34, \x{5678}   Hexadecimal character

       C's "\v" for vertical tab is not supported as it is rarely used in practice and it
       collides with Perl's "\v" regexp escape. Perl's "\c" for forming other control characters
       is also not supported.

   token_ident
          $ident = $parser->token_ident

       Expects to find an identifier, and consumes it.

   token_kw
          $keyword = $parser->token_kw( @keywords )

       Expects to find a keyword, and consumes it. A keyword is defined as an identifier which is
       exactly one of the literal values passed in.

EXAMPLES

   Accumulating Results Using Variables
       Although the structure-forming methods all return a value, obtained from their nested
       parsing code, it can sometimes be more convenient to use a variable to accumulate a result
       in instead. For example, consider the following parser method, designed to parse a set of
       "name: "value"" assignments, such as might be found in a configuration file, or
       YAML/JSON-style mapping value.

        sub parse_dict
        {
           my $self = shift;

           my %ret;
           $self->list_of( ",", sub {
              my $key = $self->token_ident;
              exists $ret{$key} and $self->fail( "Already have a mapping for '$key'" );

              $self->expect( ":" );

              $ret{$key} = $self->parse_value;
           } );

           return \%ret
        }

       Instead of using the return value from "list_of", this method accumulates values in the
       %ret hash, eventually returning a reference to it as its result. Because of this, it can
       perform some error checking while it parses; namely, rejecting duplicate keys.

TODO

       ·   Make unescaping of string constants more customisable. Possibly consider instead a
           "parse_string_generic" using a loop over "substring_before".

       ·   Easy ability for subclasses to define more token types as methods. Perhaps provide a
           class method such as

            __PACKAGE__->has_token( hex => qr/[0-9A-F]+/i, sub { hex $_[1] } );

       ·   Investigate how well "from_reader" can cope with buffer splitting across other tokens
           than simply skippable whitespace

AUTHOR

       Paul Evans <leonerd@leonerd.org.uk>