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.

 Stall Detection

    Most of the methods in this class have bounded execution time, but some
    methods ("list_of" and "sequence_of") repeatedly recuse into other code
    to build up a list of results until some ending condition is reached. A
    possible class of bug is that whatever they recurse into might
    successfully match an empty string, and thus make no progress.

    These methods will automatically detect this situation if they
    repeatedly encounter the same string position more than a certain
    number of times (given by the stallcount argument). If this count is
    reached, the entire parse attempt will be aborted by the "die" method.

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.

    stallcount => INT

      Since version 0.21.

      The number of times that the stall-detector would have to see the
      same position before it aborts the parse attempt. If not supplied, a
      default of 10 will apply.

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 /["']/.

SUBCLASSING METHODS

    The following optional methods may be defined by subclasses, to
    customise their parsing.

 on_parse_start

       $parser->on_parse_start;

    Since version 0.21.

    If defined, is invoked by the from_* method that begins a new parse
    operation, just before invoking the toplevel structure method.

 on_parse_end

       $result = $parser->on_parse_end( $result );

    Since version 0.21.

    If defined, is invoked by the from_* method once it has finished the
    toplevel structure method. This is passed the tentative result from the
    structure method, and whatever it returns becomes the result of the
    from_* method itself.

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)" );

 filename

       $filename = $parser->filename;

    Since version 0.20.

    Returns the name of the file currently being parsed, if invoked from
    within "from_file".

 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 interactively 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
    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.

 die

 die_from

       $parser->die( $message );
    
       $parser->die_from( $pos, $message );

    Since version 0.20.

    Throws an exception that propagates as normal for die, entirely out of
    the entire parser and to the caller of the toplevel from_* method that
    invoked it, bypassing all of the back-tracking logic.

    This is much like using core's die directly, except that the message
    string will include the line and column position, and the line of input
    that the parser was working on, as it does in the "fail" method.

    This method is intended for reporting fatal errors where the parsed
    input was correctly recognised at a grammar level, but is requesting
    something that cannot be fulfilled semantically.

 warn

 warn_from

       $parser->warn( $message );
    
       $parser->warn_from( $pos, $message );

    Since version 0.22.

    Issues a warning as normal for the warn core Perl function, appending
    information to the message string giving the current line and column
    position and the line of input the parser was working on, as it does in
    the "fail" method.

 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.

 include_string

       $result = $parser->include_string( $str, %opts );

    Since version 0.21.

    Parses a given string into the existing parser object.

    The current parser state is moved aside from the duration of this
    method, and is replaced by the given string. Then the toplevel parser
    method (or a different as specified) is invoked over it. Its result is
    returned by this method.

    This would typically be used to handle some sort of "include" or "macro
    expansion" ability, by injecting new content in as if the current parse
    location had encountered it. Other than the internal parser state,
    other object fields are not altered, so whatever effects the invoked
    parsing methods will have on it can continue to inspect and alter it as
    required.

    The following options are recognised:

    filename => STRING

      If set, provides a filename (or other descriptive text) to pretend
      for the source of this string. It need not be a real file on the
      filesystem; it could for example explain the source of the string in
      some other way. It is the value reported by the "filename" method and
      printed in failure messages.

    toplevel => STRING | CODE

      If set, provides the toplevel parser method to use within this
      inclusion, overriding the object's defined default.

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.

    Anywhere that a code reference is expected also permits a plain string
    giving the name of a method to invoke. This is sufficient in many
    simple cases, such as

       $self->any_of(
          'token_int',
          'token_string',
          ...
       );

 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.

       $ret = $parser->maybe( $code, @args );

    Since version 0.22 this method passes any additional arguments into the
    invoked code. This is especially useful if the $code is a method name.

    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( "{", '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, '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. It is called in list
    context, and whatever values it returns are appended to the eventual
    result - similar to perl's map.

    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( ",", '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( '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 attempted.

    If none of the choices match then a simple failure message is printed:

       Found nothing parseable

    As this is unlikely to be helpful to users, a better message can be
    provided by the final choice instead. Don't forget to commit before
    printing the failure message, or it won't count.

       $self->any_of(
          'token_int',
          'token_string',
          ...,
    
          sub { $self->commit; $self->fail( "Expected an int or string" ) }
       );

 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.

 nonempty_substring_before

       $str = $parser->nonempty_substring_before( $literal );
    
       $str = $parser->nonempty_substring_before( qr/pattern/ );

    Since version 0.20.

    A variant of "substring_before" which fails if the matched part is
    empty.

    The example above could have been written:

       sub token_nonempty_part
       {
          my $self = shift;
    
          return $parser->nonempty_substring_before( "," );
       }

    This is often useful for breaking out of repeating loops; e.g.

       sub token_escaped_string
       {
          my $self = shift;
          $self->expect( '"' );
    
          my $ret = "";
          1 while $self->any_of(
             sub { $ret .= $self->nonempty_substring_before( qr/%|$/m ); 1 }
             sub { my $escape = ( $self->expect( qr/%(.)/ ) )[1];
                   $ret .= _handle_escape( $escape );
                   1 },
             sub { 0 },
          )
    
          return $ret;
       }

 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>