parser.y

class MathParser
  # Token types from our lexer
  token tPLUS   tMINUS  tSTAR
        tDIVIDE tLPAREN tRPAREN
        tNUMBER

  # operator precedence
  prechigh
    left tSTAR tDIVIDE
    left tPLUS tMINUS
  preclow

  rule
    # exp is one of the other rules
    exp: operation
       | paren
       | number

    # return :number node
    number: tNUMBER { result = [:number, val[0]] }

    # return result between parentheses
    paren: tLPAREN exp tRPAREN { result = val[1] }

    # return :send node for all operations
    operation: exp tPLUS exp   { result = [:send, val[0], val[1].to_sym, val[2]] }
             | exp tMINUS exp  { result = [:send, val[0], val[1].to_sym, val[2]] }
             | exp tSTAR exp   { result = [:send, val[0], val[1].to_sym, val[2]] }
             | exp tDIVIDE exp { result = [:send, val[0], val[1].to_sym, val[2]] }
end

---- header
# see https://gist.github.com/skryukov/6568d7baecfe696c02f9dc3cc14240c2
require_relative "./lexer.rb"

---- inner

def parse(arg)
  @tokens = Lexer.new.run(arg)
  do_parse
end

def next_token
  @tokens.shift
end