# Wrote this in the bathtub just as a curiousity to see how hard it'd be :shrug:
require 'strscan'

def scan(str)
  tokens  = []
  scanner = StringScanner.new str
  loop do
    next if scanner.scan /\s+/
    token = scanner.scan /[()]/
    next tokens << token if token
    token = scanner.scan /[^\s()"']+/
    next tokens << token if token
    break
  end
  tokens# scanner.pos
end

def parse(str)
  tokens = scan str
  stack = [[]]
  until tokens.empty?
    current = stack.last
    token = tokens.shift
    case token
    when '('
      stack.push []
    when ')'
      current = stack.pop
      stack.last.push current
    when /\A\d+\z/
      stack.last.push token.to_f
    else
      stack.last.push token.intern
    end
  end
  raise unless stack.one?
  stack.pop
end

parse <<~LISP
(=(+(* a (square x))
    (* b x)
    c)
  0)
(= (f (x))
   (square x))
LISP
# => [[:"=", [:+, [:*, :a, [:square, :x]], [:*, :b, :x], :c], 0.0],
#     [:"=", [:f, [:x]], [:square, :x]]]