Learning racc

How to run

% racc parser.y -o parser.rb
% ruby parser.rb
1+2-3*4/5

parser.y

class Parser
rule
  expression:
      term
      | expression '+' term { puts '+'; result = val[0] + val[2] }
      | expression '-' term { puts '-'; result = val[0] - val[2] }
  term :
      primary_expression
      | term '*' primary_expression { puts '*'; result = val[0] * val[2] }
      | term '/' primary_expression { puts '-'; result = val[0] / val[2] }

  primary_expression:
      INTEGER
      | DOUBLE
end

---- inner

def parse(text)
  @scanner = StringScanner.new text
  do_parse
end

def next_token
  if @scanner.eos?
    [false, nil]
  elsif t = @scanner.scan(/\d+\.\d*/)
    [:DOUBLE, t.to_f]
  elsif t = @scanner.scan(/\d+/)
    [:INTEGER, t.to_i]
  elsif @scanner.scan(/\s+/)
    next_token
  elsif t = @scanner.scan(/[\+\-\*\/]/)
    [t, nil]
  end
end

---- footer

if $0 == __FILE__
  require 'strscan'
  $stdin.each_line do |line|
    puts Parser.new.parse(line)
  end
end