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|
# -----------------------------------------------------------------------------
# calc.py
#
# A simple calculator with variables. This is from O'Reilly's
# "Lex and Yacc", p. 63.
# -----------------------------------------------------------------------------
tokens = (
'NAME','NUMBER',
'PLUS','MINUS','TIMES','DIVIDE','EQUALS',
'LPAREN','RPAREN',
)
# Tokens
t_PLUS = r'\+'
t_MINUS = r'-'
t_TIMES = r'\*'
t_DIVIDE = r'/'
t_EQUALS = r'='
t_LPAREN = r'\('
t_RPAREN = r'\)'
t_NAME = r'[a-zA-Z_][a-zA-Z0-9_]*'
def t_NUMBER(t):
r'\d+'
try:
t.value = int(t.value)
except ValueError:
print "Integer value too large", t.value
t.value = 0
return t
t_ignore = " \t"
def t_newline(t):
r'\n+'
t.lineno += t.value.count("\n")
def t_error(t):
print "Illegal character '%s'" % t.value[0]
t.skip(1)
# Build the lexer
import lex
lex.lex()
# Parsing rules
precedence = (
('left','PLUS','MINUS'),
('left','TIMES','DIVIDE'),
('right','UMINUS'),
)
# dictionary of names
names = { }
def p_statement_assign(t):
'statement : NAME EQUALS expression'
names[t[1]] = t[3]
def p_statement_expr(t):
'statement : expression'
print t[1]
def p_expression_binop(t):
'''expression : expression PLUS expression
| expression MINUS expression
| expression TIMES expression
| expression DIVIDE expression'''
if t[2] == '+' : t[0] = t[1] + t[3]
elif t[2] == '-': t[0] = t[1] - t[3]
elif t[2] == '*': t[0] = t[1] * t[3]
elif t[2] == '/': t[0] = t[1] / t[3]
def p_expression_uminus(t):
'expression : MINUS expression %prec UMINUS'
t[0] = -t[2]
def p_expression_group(t):
'expression : LPAREN expression RPAREN'
t[0] = t[2]
def p_expression_number(t):
'expression : NUMBER'
t[0] = t[1]
def p_expression_name(t):
'expression : NAME'
try:
t[0] = names[t[1]]
except LookupError:
print "Undefined name '%s'" % t[1]
t[0] = 0
def p_error(t):
print "Syntax error at '%s'" % t.value
import yacc
yacc.yacc()
while 1:
try:
s = raw_input('calc > ')
except EOFError:
break
yacc.parse(s)
|
