语法中的“表达式”导致符号错误

Question

我一直在研究一些PLY代码，在我的解析器语法中，有一些东西我想要“含糊不清”。例如，与“打印”等价的是“显示”。我不希望show只打印特定类型，而是希望它打印任何内容，为此我尝试在语法中使用表达式，但它给了我一个错误： 这是我的密码

import ply.lex as lex
import ply.yacc as yacc


reserved = {
  # 'if' : 'IF',
  # 'then' : 'THEN',
  # 'else' : 'ELSE',
  # 'repeat-if' : 'REPEAT-IF',
  'is' : "IS",
  'show' : "SHOW",
  'receive': "RECEIVE"
}

tokens = ['LPAREN','RPAREN','STRING', 'NAME'] + list(reserved.values())

def t_ID(t):
  r'[a-zA-Z_][a-zA-Z_0-9]*'
  t.type = reserved.get(t.value,'NAME')    # Check for reserved words
  return t


t_LPAREN =r'\('
t_RPAREN = r'\)'
t_STRING = r'\".*?\"'
t_ignore = " \t"


def t_error(t):
  print("Syntax error at %s"%t.value)



Variables = {}


def p_receive(p):
  '''statement : NAME IS RECEIVE LPAREN STRING RPAREN'''
  value = input(f"{(p.slice[5].value[1:-1])}")
  Variables[p.slice[1].value]=value

def p_show(p):
  'statement : SHOW LPAREN expression RPAREN '
  value = p.slice[3]
  #TO-DO
  
def p_is(p):
  'statement : NAME IS STRING'
  Variables[p[1]] = p[3]

def p_variable(p):
  'statement : NAME'
  return Variables[p.slice[1].value]

def p_error(p):
  print("Syntax error at %s"%p)

lexer = lex.lex()
yacc.yacc()

with open('CODE.txt','r') as file:
  code = file.read().splitlines()

for line in code:
  try:
    s = line   
  except EOFError:
    break
  yacc.parse(s)

我得到以下错误：

Symbol 'expression' used, but not defined as a token or a rule

然而，使用来自https://www.dabeaz.com/ply/example.html的PLY示例，我从未看到必须定义表达式