self.lex.start = varinfo[1]
self.lex.showerror() # returns None
def interpret(self, tree):
result = tree.apply(self.vars) # tree evals itself
if result != None: # ignore 'set' result
print(result) # ignores errors
def Goal(self):
if self.lex.token in ['num', 'var', '(']:
tree = self.Expr()
self.lex.match('\0')
return tree
elif self.lex.token == 'set':
tree = self.Assign()
self.lex.match('\0')
return tree
else:
raise SyntaxError()
def Assign(self):
self.lex.match('set')
vartree = VarNode(self.lex.value, self.lex.start)
self.lex.match('var')
valtree = self.Expr()
return AssignNode(vartree, valtree) # two subtrees
def Expr(self):
left = self.Factor() # left subtree
while True:
if self.lex.token in ['\0', ')']:
return left
elif self.lex.token == '+':
self.lex.scan()
left = PlusNode(left, self.Factor()) # add root-node
elif self.lex.token == '-':
self.lex.scan()
left = MinusNode(left, self.Factor()) # grows up/right
else:
raise SyntaxError()
def Factor(self):
left = self.Term()
while True:
if self.lex.token in ['+', '-', '\0', ')']:
return left
elif self.lex.token == '*':
self.lex.scan()
left = TimesNode(left, self.Term())
elif self.lex.token == '/':
self.lex.scan()
left = DivideNode(left, self.Term())
else:
raise SyntaxError()
1452 | Chapter 19: Text and Language