Module: Racc (Ruby 2.2.3)
Description: Module : Racc - Ruby 2.2.3

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Support for the Ruby 2.2 series has ended. See here for reference.

### Home Classes Methods

### In Files

- racc/parser.rb

### Namespace

- CLASS Racc::ParseError
- CLASS Racc::Parser

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# Racc

Racc is a LALR(1) parser generator. It is written in Ruby itself, and generates Ruby programs.

## Command-line Reference¶ ↑

    racc [-o<var>filename</var>] [--output-file=<var>filename</var>]
         [-e<var>rubypath</var>] [--embedded=<var>rubypath</var>]
         [-v] [--verbose]
         [-O<var>filename</var>] [--log-file=<var>filename</var>]
         [-g] [--debug]
         [-E] [--embedded]
         [-l] [--no-line-convert]
         [-c] [--line-convert-all]
         [-a] [--no-omit-actions]
         [-C] [--check-only]
         [-S] [--output-status]
         [--version] [--copyright] [--help] <var>grammarfile</var>

`filename`

  Racc grammar file. Any extension is permitted.

-o+outfile+, –output-file=`outfile`

  A filename for output. default is <`filename`>.tab.rb

-O+filename+, –log-file=`filename`

  Place logging output in file `filename`. Default log file name is <`filename`>.output.

-e+rubypath+, –executable=`rubypath`

  output executable file(mode 755). where `path` is the Ruby interpreter.

-v, –verbose

  verbose mode. create `filename`.output file, like yacc's y.output file.

-g, –debug

  add debug code to parser class. To display debugging information, use this '-g' option and set @yydebug true in parser class.

-E, –embedded

  Output parser which doesn't need runtime files (racc/parser.rb).

-C, –check-only

  Check syntax of racc grammar file and quit.

-S, –output-status

  Print messages time to time while compiling.

-l, –no-line-convert

  turns off line number converting.

-c, –line-convert-all

  Convert line number of actions, inner, header and footer.

-a, –no-omit-actions

  Call all actions, even if an action is empty.

–version

  print Racc version and quit.

–copyright

  Print copyright and quit.

–help

  Print usage and quit.

## Generating Parser Using Racc¶ ↑

To compile Racc grammar file, simply type:

    $ racc parse.y

This creates Ruby script file “parse.tab.y”. The -o option can change the output filename.

## Writing A Racc Grammar File¶ ↑

If you want your own parser, you have to write a grammar file. A grammar file contains the name of your parser class, grammar for the parser, user code, and anything else. When writing a grammar file, yacc's knowledge is helpful. If you have not used yacc before, Racc is not too difficult.

Here's an example Racc grammar file.

    class Calcparser
    rule
      target: exp { print val[0] }

      exp: exp '+' exp
         | exp '*' exp
         | '(' exp ')'
         | NUMBER
    end

Racc grammar files resemble yacc files. But (of course), this is Ruby code. yacc's $$ is the 'result', $0, $1… is an array called 'val', and $-1, $-2… is an array called '_values'.

See the Grammar File Reference for more information on grammar files.

## Parser¶ ↑

Then you must prepare the parse entry method. There are two types of parse methods in Racc, Racc::Parser#do_parse and Racc::Parser#yyparse

Racc::Parser#do_parse is simple.

It's yyparse() of yacc, and Racc::Parser#next_token is yylex(). This method must returns an array like [TOKENSYMBOL, ITS_VALUE]. EOF is [false, false]. (TOKENSYMBOL is a Ruby symbol (taken from String#intern) by default. If you want to change this, see the grammar reference.

Racc::Parser#yyparse is little complicated, but useful. It does not use Racc::Parser#next_token, instead it gets tokens from any iterator.

For example, `yyparse(obj, :scan)` causes calling +obj#scan+, and you can return tokens by yielding them from +obj#scan+.

## Debugging¶ ↑

When debugging, “-v” or/and the “-g” option is helpful.

“-v” creates verbose log file (.output). “-g” creates a “Verbose Parser”. Verbose Parser prints the internal status when parsing. But it's not automatic. You must use -g option and set +@yydebug+ to `true` in order to get output. -g option only creates the verbose parser.

### Racc reported syntax error.¶ ↑

Isn't there too many “end”? grammar of racc file is changed in v0.10.

Racc does not use '%' mark, while yacc uses huge number of '%' marks..

### Racc reported “XXXX conflicts”.¶ ↑

Try “racc -v xxxx.y”. It causes producing racc's internal log file, xxxx.output.

### Generated parsers does not work correctly¶ ↑

Try “racc -g xxxx.y”. This command let racc generate “debugging parser”. Then set @yydebug=true in your parser. It produces a working log of your parser.

## Re-distributing Racc runtime¶ ↑

A parser, which is created by Racc, requires the Racc runtime module; racc/parser.rb.

Ruby 1.8.x comes with Racc runtime module, you need NOT distribute Racc runtime files.

If you want to include the Racc runtime module with your parser. This can be done by using '-E' option:

    $ racc -E -omyparser.rb myparser.y

This command creates myparser.rb which `includes' Racc runtime. Only you must do is to distribute your parser file (myparser.rb).

Note: parser.rb is LGPL, but your parser is not. Your own parser is completely yours.