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->one_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 atomically extract a grammatical token from the string. It also provides wrapping methods that can be used to build up a possibly-recursive grammar structure. Each method, both token and structural, atomically either consumes a prefix of the string and returns its result, or fails and consumes nothing. CONSTRUCTOR $parser = Parser::MGC->new( %args ) Returns a new instance of a "Parser::MGC" object. This must be called on a subclass that provides a "parse" method. Takes the following named arguments 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 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. * 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 $result = $parser->from_string( $str ) Parse the given literal string and return the result from the "parse" method. $result = $parser->from_file( $file ) Parse the given file, which may be a pathname in a string, or an opened IO handle, and return the result from the "parse" method. ( $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. $parser->fail( $message ) Aborts the current parse attempt with the given message string. The failure message will include the current line and column position, and the line of input that failed. $eos = $parser->at_eos Returns true if the input string is at the end of the string. 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. $ret = $parser->maybe( $code ) Attempts to execute the given $code reference in scalar context, and returns what it returned. If the code fails to parse by calling the "fail" method then none of the input string will be consumed; the current parsing position will be restored. "undef" will be returned in this case. 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 } ), ]; } $ret = $parser->scope_of( $start, $code, $stop ) Expects to find the $start pattern, then attempts to execute the given $code reference, then expects to find the $stop pattern. Returns whatever the code reference returned. 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 }, "}" ); } $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. 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 } ); } $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 } ); } $ret = $parser->one_of( @codes ) Expects that one of the given code references can parse something from the input, returning what it returned. Each code reference may indicate a failure to parse by calling the "fail" method. 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->one_of( sub { $self->parse_declaration; $self->expect(";") }, sub { $self->parse_expression; $self->expect(";") }, sub { $self->parse_block }, ); } $parser->commit Calling this method will cancel the backtracking behaviour of the innermost "maybe" or "one_of" structure forming method. That is, if later code then calls "fail", the exception will be propagated out of "maybe", and no further code blocks will be attempted by "one_of". Typically this will be called once the grammatical structure of an alternation 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->one_of( ... sub { $self->scope_of( "{", sub { $self->commit; $self->parse_statements; }, "}" ), }, ); } TOKEN PARSING METHODS The following methods attempt to consume some part of the input string, to be used as part of the parsing process. $parser->expect( $string ) $parser->expect( qr/pattern/ ) Expects to find a literal string or regexp pattern match, and consumes it. This method returns the string that was captured. $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. $str = $parser->token_string Expects to find a quoted string, and consumes it. The string should be quoted using """ or "'" quote marks. $ident = $parser->token_ident Expects to find an identifier, and consumes it. $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. TODO * Unescaping of string constants; customisable * Easy ability for subclasses to define more token types AUTHOR Paul Evans