"""
    sample use:

    l.eval(("seq", ("set", "n", 7),
                   ("set", "f", 1),
                   ("while", (">", "n", 1),
                             ("seq", ("set", "f", ("*", "f", "n")),
                                     ("set", "n", ("-", "n", 1)))),
                   ("print", "f")))
    5040    

"""

import functools, operator

arith = { "+": operator.add, "-": operator.sub, "*": operator.mul, "/": operator.truediv,
          "%": operator.mod, "^": operator.pow, 
          "and": operator.and_, "or": operator.or_, "xor": operator.xor }
compa = { "==": operator.eq, "<": operator.lt, ">": operator.gt,
          "!=": operator.ne, "<=": operator.le, ">=": operator.ge }

mem = {}

def reduce(f, vals):
  v = vals[0]
  for x in vals[1:]:
    v = f(v, x)
  return v

def eval(form):

  if type(form) == int or type(form) == float:
    return form

  if type(form) == str:
    if form in mem:
      return mem[form]
    else:
      raise ValueError("unknown variable " + form)

  if type(form) == tuple:
    if form == ():
      raise ValueError("empty")
    op = form[0]

    if op in arith:
      return reduce(arith[op], tuple((eval(f) for f in form[1:])))

    if op in compa:
      return compa[op](eval(form[1]), eval(form[2]))
      
    if op == "neg":   
      v = eval(form[1])
      if type(v) == bool:
        return not v
      if type(v) == int:
        return - v
      raise ValueError("wrong type for neg")

    if op == "set":
      mem[form[1]] = eval(form[2])
      return mem[form[1]]

    if op == "seq":
      v = 0
      for f in form[1:]:
        v = eval(f)
      return v

    if op == "print":
      v = eval(form[1])
      print(v)
      return v

    if op == "if":
      if eval(form[1]):
        return eval(form[2])
      else:
        return eval(form[3])

    if op == "while":
      v = 0
      while eval(form[1]):
        v = eval(form[2])
      return v

    if op == "read":
      while True:
        return readvalue()

  raise ValueError("unknown " + str(form))

def readvalue():
  while True:
    x = input("? ")
    if x:      
      break
  if x == "true":
    return True
  if x == "false":
    return False
  if x.isdecimal():
    return int(x)
  return x

f1 = ("*", 3, 4, 5)
f2 = ("*", ("+", 1, 2), ("*", 2, 2), ("+", 1, ("*", 2, 2)))
f3 = ("<", f1, f2)
f4 = ("set", "x", f2)
f5 = ("seq", ("set", "x", 17), ("set", "y", 3), ("print", "x"), ("*", "x", "y"))