Getting started with racc (ruby LALR(1) parser lib)

grammar.y.rb

class Calcparser
  prechigh
    left     '*' '/'
    left     '+' '-'
  preclow

  options no_result_var

  rule
  target: expr
  expr: expr "+" expr { [:+, val[0], val[2]] }
      | expr "*" expr { [:*, val[0], val[2]] }
      | NUMBER
end

---- inner
def make_tokens str
  require 'strscan'

  result = []
  scanner = StringScanner.new str
  until scanner.empty?
    case
      when scanner.scan(/\s+/)
      #ignore whitespace
      when match = scanner.scan(/\+/)
        result << ["+", "+"]
      when match = scanner.scan(/\*/)
        result << ["*", "*"]
      when match = scanner.scan(/\d+/)
        result << [:NUMBER, match[0].to_i]
      else
        raise "can't recognize  <#{scanner.peek(5)}>"
    end
  end
  result << [false, false]
  return result
end

attr_accessor :result

def parse(str)
  @result = []
  @tokens = make_tokens str
  do_parse
end

def next_token
  @tokens.shift
end

main.rb

require './grammar.tab'

parser = Calcparser.new
x = parser.parse "1 + 2 * 1"
p x

run.sh

racc grammar.y.rb; echo; ruby main.rb
# output : [:+, 1, [:*, 2, 1]]