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Marpa-R2-2.043_002
River stage two • 39 direct dependents • 56 total dependents
48 ++
/ Marpa::R2::Scanless::G

Name

Marpa::R2::Scanless::G - Scanless interface grammars

Synopsis

    my $grammar = Marpa::R2::Scanless::G->new(
        {   
            action_object  => 'My_Actions',
            default_action => 'do_first_arg',
            source          => \(<<'END_OF_SOURCE'),
    :start ::= Script
    Script ::= Expression+ separator => comma action => do_script
    comma ~ [,]
    Expression ::=
        Number
        | '(' Expression ')' action => do_parens assoc => group
       || Expression '**' Expression action => do_pow assoc => right
       || Expression '*' Expression action => do_multiply
        | Expression '/' Expression action => do_divide
       || Expression '+' Expression action => do_add
        | Expression '-' Expression action => do_subtract
    Number ~ [\d]+

    :discard ~ whitespace
    whitespace ~ [\s]+
    # allow comments
    :discard ~ <hash comment>
    <hash comment> ~ <terminated hash comment> | <unterminated
       final hash comment>
    <terminated hash comment> ~ '#' <hash comment body> <vertical space char>
    <unterminated final hash comment> ~ '#' <hash comment body>
    <hash comment body> ~ <hash comment char>*
    <vertical space char> ~ [\x{A}\x{B}\x{C}\x{D}\x{2028}\x{2029}]
    <hash comment char> ~ [^\x{A}\x{B}\x{C}\x{D}\x{2028}\x{2029}]
    END_OF_SOURCE
        }
    );

About this document

This page is the reference for the grammar objects of Marpa's Scanless interface.

Constructor

The new() method is the constructor for Scanless grammars. An example of its use is above. The new() constructor accepts a hash of named arguments. The following named arguments are allowed:

action_object

Specifies the action_object named argument that will be used for the G1 grammar. For details, see "action_object" in Marpa::R2::Grammar.

default_action

Specifies the default_action named argument that will be used for the G1 grammar. For details, see "default_action" in Marpa::R2::Grammar.

source

The value of the source named argument must be a reference to a string which contains a description of the grammar. The string's format is described below.

trace_file_handle

The value is a file handle. Trace output and warning messages go to the trace file handle. By default the trace file handle is STDERR.

Trace method

show_rules()

    my $show_rules_output = $grammar->show_rules();

The show_rules() method is the equivalent of calling Marpa::R2::Grammar's show_rules() method for both the G0 and G1 grammars. It is useful for showing the rules as they will appear in trace and debugging outputs.

Source strings

The format of Scanless source strings is that of BNF source strings, with the following changes and extensions:

Structural rules

The BNF operator ("::=") has the same function as before, but in addition it indicates that the rule is a G1 rule. Start rules must be G1 rules.

Lexical rules

The match operator ("~") can be used between the LHS and RHS of a rule, instead of the BNF operator. Rules which use the match operator are G0 rules. Discard rules must be G0 rules. A lexical rule cannot have an action adverb.

Discard rules

A discard rule is a rule whose LHS is the :discard pseudo-symbol, and whose RHS is a single symbol name, called the discarded symbol. These rules indicate that the discarded symbol is a top-level G0 symbol, but one which is not a lexeme. When a discarded symbol is recognized, it is not passed as a lexeme to the G1 parser, but is (as the name suggests) discarded.

Single quoted strings

Single quotes can be used in prioritized rules to indicate character strings. The characters inside the single quote will be matched in the input, literally and one-for-one. Single quoted strings can contain any characters with the exception of single quotes and vertical whitespace.

Single quoted strings do not allow "escaped" characters. A backslash ("\") represents itself and has no effect on the interpretation of the next character. If a rule needs to match one of the forbidden characters (single quote or vertical whitespace), it must use a character class.

Single quoted strings are always interpreted at the G0 level, but they may be used in either structural or lexical rules. When a single quoted string is used in a structural rule, Marpa creates a virtual G0 rule on behalf of the application. This is handy, but it does have a real disadvantage -- the name of the virtual rule's LHS will be one assigned automatically by Marpa. When tracing and debugging parses and grammars, these virtual LHS's can be harder for a programmer to interpret.

Character classes

A character class in square brackets ("[]") can be used in a RHS alternative of a prioritized rule, or on the RHS of a quantified rule or a discard rule. Marpa character classes may contain anything acceptable to Perl, and follow the same escaping conventions as Perl's character classes.

Character classes are always interpreted at the G0 level, but they may be used in either structural or lexical rules. When a character class is used in a structural rule, Marpa creates a virtual G0 rule on behalf of the application. This is handy, but it does have a real disadvantage -- the name of the virtual rule's LHS will be one assigned automatically by Marpa. When tracing and debugging parses and grammars, these virtual LHS's can be harder for a programmer to interpret.

An implementation note: character classes are interpreted by Perl, but this involves minimal overhead when the parse is of any length. Each character class is passed to Perl to interpret exactly once and the result is memoized in a C structure for future use.

Copyright and License

  Copyright 2013 Jeffrey Kegler
  This file is part of Marpa::R2.  Marpa::R2 is free software: you can
  redistribute it and/or modify it under the terms of the GNU Lesser
  General Public License as published by the Free Software Foundation,
  either version 3 of the License, or (at your option) any later version.

  Marpa::R2 is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser
  General Public License along with Marpa::R2.  If not, see
  http://www.gnu.org/licenses/.