sexp_parser.rb

class SexpParser
  def initialize
    @tokens = []
  end

  def parse_list
    xs = []
    xs << parse_atom until @tokens.fetch(0) == :')'
    @tokens.shift
    xs
  end

  def parse_atom
    x = @tokens.shift
    x == :'(' ? parse_list : x
  end

  def parse(text)
    @tokens = text.scan(/[()]|[^()\s]+/)
    @tokens.map! { |x| x =~ /\A\d+\z/ ? x.to_i : x.to_sym }
    # p @tokens
    parse_atom
  end
end
parser = SexpParser.new
p parser.parse '(+ (- a b) c)'
p parser.parse '(define (f x) (+ x 1))'