CODEWITHSUNDEEP
python-3.xply
user10149539
user10149539

Reputation:

Python Lex-Yacc reports "Infinite recursion detected for symbol"

I have a lot of error using PLY. The error happens when I try to add this to the code:

def p_assign(p):
   '''assign : NAME EQUALS expr'''
    p[0] = ('ASSIGN',p[1],p[3])
def p_expr_plus(p):
    '''expr : expr PLUS term'''
    p[0] = ('+',p[1],p[3])
def p_term_mul(p):
    '''term : term TIMES factor'''
    p[0] = ('*',p[1],p[3])
def p_term_factor(p):
    '''term : factor'''
    p[0] = p[1]
def p_factor(p):
    '''factor : NUMBER'''
    p[0] = ('NUM',p[1])

But I get this error:

ERROR: C:\Users\-user-\Desktop\dw_parser.py:9: Symbol 'EQUALS' used, but not defined as a token or a rule
WARNING: Token 'COLON' defined, but not used
WARNING: Token 'DIVIDE' defined, but not used
WARNING: Token 'EQUAL' defined, but not used
WARNING: Token 'EQUAL_TO' defined, but not used
WARNING: Token 'GREATER' defined, but not used
WARNING: Token 'GREATER_EQL' defined, but not used
WARNING: Token 'LESS' defined, but not used
WARNING: Token 'LESS_EQL' defined, but not used
WARNING: Token 'LPAREN' defined, but not used
WARNING: Token 'MINUS' defined, but not used
WARNING: Token 'QUOTATION' defined, but not used
WARNING: Token 'REMAINDER' defined, but not used
WARNING: Token 'RPAREN' defined, but not used
WARNING: Token 'SEMICOLON' defined, but not used
WARNING: Token 'STRING' defined, but not used
WARNING: There are 15 unused tokens
ERROR: Infinite recursion detected for symbol 'assign'
ERROR: Infinite recursion detected for symbol 'expr'
Traceback (most recent call last):
  File "C:\Users\-user-\Desktop\dw_parser.py", line 35, in <module>
    parser = yacc.yacc()
  File "C:\Program Files\Python37\lib\site-packages\ply-3.11-py3.7.egg\ply\yacc.py", line 3432, in yacc
    raise YaccError('Unable to build parser')
ply.yacc.YaccError: Unable to build parser

There is the full parser code: https://pastebin.com/bPrwDQEK I can't put the parser code directly here, because code sample doesn't works too.

Upvotes: 0

Views: 3107

Answers (1)

rici
rici

Reputation: 241721

The only production you define for expr is

expr : expr PLUS term

Since there is no other production, expr cannot ever produce a finite sentence. Contrast this with your productions for term:

term : factor TIMES term
term : factor

Here, there is a non-recursive production so the recursion can terminate.

Because of the way PLY works, you do not need a separate parsing action function to add the expr : term production; you can use the same function as for term : factor (and for other similar unit productions):

def p_unit(p):
    '''expr : term
       term : factor
    '''
    p[0] = p[1]

assign is also flagged because its only production uses expr and expr only leads to infinite recursion. Hopefully the other errors and warnings are self-explanatory.

Upvotes: 2

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