Disallowing ?:?: syntax

[BCS]

Reply to Miles,

> BCS wrote:
> 
>> I think not.
>> 
>> x ? y : a ? b : c => (x ? y : a) ? b : c
>> 
>> or
>> 
>> x ? y : a ? b : c => x ? y : (a ? b : c)
>> 
>> without checking the actual syntax you can't tell which of the above
>> will be used and (according to bearophile) if ?: followed after
>> +/-/etc the first would be.
>> 
> It simply can't be the first, due to the kind of parser used for C and
> most of its derived languages (D included). When a '?' is found, the
> parser recurses until it finds a ':' (it gets stuck in a branch of the
> syntax tree until a colon token is found).
> 
> This is in how the language is defined
> (http://www.digitalmars.com/d/2.0/expression.html):
>   ConditionalExpression:
>      OrOrExpression
>      OrOrExpression ? Expression : ConditionalExpression
> So, the third operand to the ternary operator is a
> ConditionalExpression itself, the parser have no reason to finish this
> evaluation branch if it finds another '?', it naturally recurses.
> 
> So, x ? y : a ? b : c => x ? y : (a ? b : c).
> 

You have just proven my exact point. To find how it parses you need to dig 
up the syntax, as you just did (Note I said that you can't tell how it's 
parsed *without* doing that).

Also note that most (or all) of the other operators in C/C++/C#/D/etc go 
the other way:

OrOrExpression:
   AndAndExpression
   OrOrExpression || AndAndExpression

> But I think that I know what kind of ambiguity you are talking about
> now... For me, ambiguity is something like the <...> C++ template
> definition/instantiation operator, or the function declaration/object
> variable definition ambiguity.
> 

Yes, I think you've spotted the point. I'm referring to ambiguities that 
will cause problems with <joke>I-BAL</joke> type parsers.