yacc_badtok.py revision 4479 
1# -----------------------------------------------------------------------------
2# yacc_badtok.py
3#
4# A grammar, but tokens is a bad datatype
5# -----------------------------------------------------------------------------
6
7import sys
8sys.tracebacklimit = 0
9
10sys.path.insert(0,"..")
11import ply.yacc as yacc
12
13tokens = "Hello"
14
15# Parsing rules
16precedence = (
17    ('left','PLUS','MINUS'),
18    ('left','TIMES','DIVIDE'),
19    ('right','UMINUS'),
20    )
21
22# dictionary of names
23names = { }
24
25def p_statement_assign(t):
26    'statement : NAME EQUALS expression'
27    names[t[1]] = t[3]
28
29def p_statement_expr(t):
30    'statement : expression'
31    print t[1]
32
33def p_expression_binop(t):
34    '''expression : expression PLUS expression
35                  | expression MINUS expression
36                  | expression TIMES expression
37                  | expression DIVIDE expression'''
38    if t[2] == '+'  : t[0] = t[1] + t[3]
39    elif t[2] == '-': t[0] = t[1] - t[3]
40    elif t[2] == '*': t[0] = t[1] * t[3]
41    elif t[3] == '/': t[0] = t[1] / t[3]
42
43def p_expression_uminus(t):
44    'expression : MINUS expression %prec UMINUS'
45    t[0] = -t[2]
46
47def p_expression_group(t):
48    'expression : LPAREN expression RPAREN'
49    t[0] = t[2]
50
51def p_expression_number(t):
52    'expression : NUMBER'
53    t[0] = t[1]
54
55def p_expression_name(t):
56    'expression : NAME'
57    try:
58        t[0] = names[t[1]]
59    except LookupError:
60        print "Undefined name '%s'" % t[1]
61        t[0] = 0
62
63def p_error(t):
64    print "Syntax error at '%s'" % t.value
65
66yacc.yacc()
67
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71