ANTLR for Ruby

The latest release of ANTLR v3 adds support for Ruby as a target language, courtesy of yours truly. Let's look at an example to see what's possible.

The following grammar generates a parser that can evaluate simple arithmetic expressions involving integer numbers, +, -, *, /, ( and ).

grammar Calculator;
options {
  language = Ruby;
}

@members {
  @stack = []

  def result
    @stack[0]
  end
}

parse: expression;

expression: mult (
    '+' mult {
      @stack.push(@stack.pop + @stack.pop)
    }
  | '-' mult {
      a = @stack.pop
      b = @stack.pop
      @stack.push(b - a)
    }
  )* ;

mult: atom (
    '*' atom {
      @stack.push(@stack.pop * @stack.pop)
    }
  | '/' atom {
      a = @stack.pop
      b = @stack.pop
      @stack.push(b / a.to_f)
    }
  )* ;


atom: n=NUMBER { @stack.push($n.text.to_i) }
  | '(' expression ')';

NUMBER: ('0'..'9')+;

WS: (' ' | '\n' | '\t')+ { channel = 99 };

First, we need to process the grammar with ANTLR. You'll need to have Java installed in order to do this. ANTLR will generate two files: Calculator.rb and CalculatorLexer.rb.

You'll also need the ANTLR ruby runtime library (just a small .rb file) which you can get here. Just drop into the same directory as the two .rb files created above (in the future I may package it as a gem).

We got our parser, so let's try it out. Create another ruby file with the following:

require 'Calculator'
require 'CalculatorLexer'

lexer = CalculatorLexer.new(ANTLR::CharStream.new(STDIN))
parser = Calculator.new(ANTLR::TokenStream.new(lexer))
parser.parse

puts parser.result

Now, run the program, type an expression and press CTRL-D (or CTRL-Z if in Windows) to see the results, like so:

(5 + 4) * (10 - 2) / 3
^Z
24.0

Simple... but cool, eh?

ANTLR v3 is still in pre-alpha stage, and the Ruby backend is a work in progress. So far, it can handle syntactic and semantic predicates, token labels, etc. and I'm planning to add support for scopes and token/rule parameters and return values next. AST creation will come after that.