r177435 - Objective-C [qoi]: Provide improved parse diagnostics when

[Douglas Gregor]

On Mar 19, 2013, at 2:58 PM, jahanian <fjahanian at apple.com> wrote:

> 
> On Mar 19, 2013, at 2:29 PM, Jordan Rose <jordan_rose at apple.com> wrote:
> 
>> 
>> On Mar 19, 2013, at 14:21 , jahanian <fjahanian at apple.com> wrote:
>> 
>>> 
>>> On Mar 19, 2013, at 1:42 PM, Jordan Rose <jordan_rose at apple.com> wrote:
>>> 
>>>> 
>>>> Would it make sense to not just continue but to pretend instead that the '}' is there? I feel like that will give us better recovery. (You'd eliminate those last two errors: "expected '}'" and "missing '@end'".)
>>> 
>>> I think by 'pretending' you mean insert a '}' before @end and continue. I thought about this but  I did not want to introduce the bookkeeping
>>> overhead for the correct case. Do you have a suggestion how to pretend without adding this overhead?
>> 
>> Hm, I see what you mean—right now we unilaterally consume a closing brace when we exit the loop. The easiest way to solve this is to extract the parsing loop and the T.consumeClose() into a helper function, with an early return for the @end case...but that's not exactly pretty. I guess I'll let you make the call (or stand by the call you already made).
> 
> Problem is not avoiding calling of T.consumeClose() in the incorrect case. Problem is that '@' and 'end" are two tokens and we have consumed
> '@' already  (this will cause parse error later).  I see two solutions:
> 1. When seeing '@', lookahead for 'end' and exit the loop while skipping T.consumeClose() . I think this is what you are suggesting. But it involves unnecessary overhead for the common case.
> 2. If it is possible,  I can insert a '}' two tokens before current token and reset the lexer to the inserted '}'  for the incorrect case, then problem is solved without introducing any undue overhead for
>    the common case.


You don't need to insert the '}' token. If you've consumed the '@' and see something that doesn't belong in a list of instance variables (e.g., 'property', 'end'), break out of the loop without calling consumeClose() and then PP.EnterToken() the '@' to recover.

	- Doug
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