gistfile1.rb

      class Calculator < Omerta::Grammar
        def initialize
          @vars = {}
        end

        rule :space => ' '

        rule(:var => "<letter>:x <space>*") { |x| x }

        rule :num do
          alt("<num>:n <digit>:d")  { |d| FIXME! }
          alt("<digit>:d <space>*") { |d| FIXME! }
        end

        rule :pri_expr do
          alt("<var>:x") { |x| @vars[x] }
          alt("<num>:n") { |n| n }
          alt("'(' <space>* <expr>:r ')' <space>*") { |r| r }
        end

        rule :mul_expr do
          alt("<mul_expr>:x '*' <space>* <pri_expr>:y") { |x, y| x * y }
          alt("<mul_expr>:x '/' <space>* <pri_expr>:y") { |x, y| x / y }
          alt(:pri_expr)
        end

        rule :add_expr do
          alt("<add_expr>:x '+' <space>* <mul_expr>:y") { |x, y| x + y }
          alt("<add_expr>:x '-' <space>* <mul_expr>:y") { |x, y| x - y }
          alt(:mul_expr)
        end

        rule :expr do
          alt("<var>:x '=' <space>* <expr>:r") { |x, r| @vars[x] = r }
          alt(:add_expr)
        end

        rule(:parse => "<space>* <expr>:r '\n'") { |r| p r }
      end