parser: improve infix error checking

(spotted by timeless)

mercurial/parser.py

@@ -1,91 +1,91 b''
 # parser.py - simple top-down operator precedence parser for mercurial
 #
 # Copyright 2010 Matt Mackall <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 # see http://effbot.org/zone/simple-top-down-parsing.txt and
 # http://eli.thegreenplace.net/2010/01/02/top-down-operator-precedence-parsing/
 # for background
 
 # takes a tokenizer and elements
 # tokenizer is an iterator that returns type, value pairs
 # elements is a mapping of types to binding strength, prefix and infix actions
 # an action is a tree node name, a tree label, and an optional match
 # __call__(program) parses program into a labelled tree
 
 import error
 
 class parser(object):
     def __init__(self, tokenizer, elements, methods=None):
         self._tokenizer = tokenizer
         self._elements = elements
         self._methods = methods
     def _advance(self):
         'advance the tokenizer'
         t = self.current
         try:
             self.current = self._iter.next()
         except StopIteration:
             pass
         return t
     def _match(self, m, pos):
         'make sure the tokenizer matches an end condition'
         if self.current[0] != m:
             raise error.ParseError("unexpected token: %s" % self.current[0],
                                    self.current[2])
         self._advance()
     def _parse(self, bind=0):
         token, value, pos = self._advance()
         # handle prefix rules on current token
         prefix = self._elements[token][1]
         if not prefix:
             raise error.ParseError("not a prefix: %s" % token, pos)
         if len(prefix) == 1:
             expr = (prefix[0], value)
         else:
             if len(prefix) > 2 and prefix[2] == self.current[0]:
                 self._match(prefix[2], pos)
                 expr = (prefix[0], None)
             else:
                 expr = (prefix[0], self._parse(prefix[1]))
                 if len(prefix) > 2:
                     self._match(prefix[2], pos)
         # gather tokens until we meet a lower binding strength
         while bind < self._elements[self.current[0]][0]:
             token, value, pos = self._advance()
             e = self._elements[token]
             # check for suffix - next token isn't a valid prefix
             if len(e) == 4 and not self._elements[self.current[0]][1]:
                 suffix = e[3]
                 expr = (suffix[0], expr)
             else:
                 # handle infix rules
-                infix = self._elements[token][2]
+                if len(e) < 3 or not e[2]:
+                    raise error.ParseError("not an infix: %s" % token, pos)
+                infix = e[2]
                 if len(infix) == 3 and infix[2] == self.current[0]:
                     self._match(infix[2], pos)
                     expr = (infix[0], expr, (None))
                 else:
-                    if not infix[0]:
-                        raise error.ParseError("not an infix: %s" % token, pos)
                     expr = (infix[0], expr, self._parse(infix[1]))
                     if len(infix) == 3:
                         self._match(infix[2], pos)
         return expr
     def parse(self, message):
         'generate a parse tree from a message'
         self._iter = self._tokenizer(message)
         self.current = self._iter.next()
         return self._parse()
     def eval(self, tree):
         'recursively evaluate a parse tree using node methods'
         if not isinstance(tree, tuple):
             return tree
         return self._methods[tree[0]](*[self.eval(t) for t in tree[1:]])
     def __call__(self, message):
         'parse a message into a parse tree and evaluate if methods given'
         t = self.parse(message)
         if self._methods:
             return self.eval(t)
         return t