yacc_uprec.py revision 2632:1bb2f91485ea 
1# -----------------------------------------------------------------------------
2# yacc_uprec.py
3#
4# A grammar with a bad %prec specifier
5# -----------------------------------------------------------------------------
6import sys
7sys.tracebacklimit = 0
8
9from calclex import tokens
10
11# Parsing rules
12
13# dictionary of names
14names = { }
15
16def p_statement_assign(t):
17    'statement : NAME EQUALS expression'
18    names[t[1]] = t[3]
19
20def p_statement_expr(t):
21    'statement : expression'
22    print t[1]
23
24def p_expression_binop(t):
25    '''expression : expression PLUS expression
26                  | expression MINUS expression
27                  | expression TIMES expression
28                  | expression DIVIDE expression'''
29    if t[2] == '+'  : t[0] = t[1] + t[3]
30    elif t[2] == '-': t[0] = t[1] - t[3]
31    elif t[2] == '*': t[0] = t[1] * t[3]
32    elif t[3] == '/': t[0] = t[1] / t[3]
33
34def p_expression_uminus(t):
35    'expression : MINUS expression %prec UMINUS'
36    t[0] = -t[2]
37
38def p_expression_group(t):
39    'expression : LPAREN expression RPAREN'
40    t[0] = t[2]
41
42def p_expression_number(t):
43    'expression : NUMBER'
44    t[0] = t[1]
45
46def p_expression_name(t):
47    'expression : NAME'
48    try:
49        t[0] = names[t[1]]
50    except LookupError:
51        print "Undefined name '%s'" % t[1]
52        t[0] = 0
53
54def p_error(t):
55    print "Syntax error at '%s'" % t.value
56
57import yacc
58yacc.yacc()
59
60
61
62
63