lisp.rb

# Here's a simple lisp expression parser that generates
# an abstract syntax tree in the form of nested arrays

def tokenize(lisp_expression)
  lisp_expression.gsub("(", " ( ").gsub(")", " ) ").split(" ")
end

def parse(tokens)
  raise "No tokens to parse" if tokens.empty?

  token = tokens.shift
  if token == "("
    list = []
    while tokens[0] != ")"
      list << parse(tokens)
    end
    tokens.shift
    list
  else
    atom(token)
  end
end

# an atom is "atomic" in that in can't be split up any further.
# We turn it into one of two concrete types: integer or symbol.
def atom(token)
  begin
    Integer(token)
  rescue ArgumentError
    token.to_sym
  end
end

def evaluate(expression)
  # We'll implement this together!
end

lisp_expression = "(first (list 1 (+ 2 3) 9))"
tokens = tokenize(lisp_expression)
parsed_expression = parse(tokens)

print parsed_expression