Why no (auto foo = bar) in while loops?

[Steven Schveighoffer]

On Wed, 24 Aug 2011 19:01:31 -0400, Timon Gehr <timon.gehr at gmx.ch> wrote:

> On 08/25/2011 12:47 AM, Mafi wrote:
>> Am 24.08.2011 21:04, schrieb Timon Gehr:
>>> On 08/24/2011 08:04 PM, Andrej Mitrovic wrote:
>>>> Here's some code that iterates through "parents" of some class object
>>>> until it finds an object with no parent (where parent is null):
>>>>
>>>> import std.stdio;
>>>>
>>>> class Foo
>>>> {
>>>> Foo parent;
>>>> int state;
>>>>
>>>> this (int state) { this.state = state; }
>>>> }
>>>>
>>>> void main()
>>>> {
>>>> auto foo = new Foo(0);
>>>> foo.parent = new Foo(1);
>>>> foo.parent.parent = new Foo(2);
>>>>
>>>> while (true)
>>>> {
>>>> if (auto par = foo.parent)
>>>> {
>>>> writeln(par.state);
>>>> foo = par;
>>>> }
>>>> else
>>>> {
>>>> break;
>>>> }
>>>> }
>>>> }
>>>>
>>>> (syntax-highlighted: http://codepad.org/8yHRmICh)
>>>>
>>>> But I was hoping I could simplify this by doing:
>>>>
>>>> while (auto par = foo.parent)
>>>> {
>>>> writeln(par.state);
>>>> foo = par;
>>>> }
>>>>
>>>> However that doesn't work, I get back:
>>>> expression expected, not 'auto'
>>>>
>>>> Is there a limitation on why this couldn't work or can this be added
>>>> to the language?
>>>
>>> Afaics, this could be added just like it could be added for if. In fact
>>> the compiler should be able to simply rewrite it to your first example.
>>> I think it is worth an enhancement request, because there are  
>>> situations
>>> where this would be useful, and implementation should be trivial, if
>>> somebody has the time. (I also think it adds to the consistency of the
>>> language, but others may disagree.)
>>>
>>>
>>> (btw, i always use for(;;) instead of while(true), it is usually faster
>>> in debug mode and faster to type :))
>>
>> I just have to say that it already works for 'if'. It's a great feature
>> because in the body of the 'if' you know the value is non-null and
>> outside, where there could be an segfault, the variable is not visible.
>> I'm not really sure if it's good for 'while'.
>> I'm unsure because there are two somewhat natural semantics for such a
>> construct.
>>
>> //example
>> //more to the nature of while
>> while(auto obj = init) { work(obj); }
>>
>> //1 (your interpretation)
>> typeof(init) obj;
>> while(obj = init) { work(obj); }
>>
>> //2
>> /*
>> seems more natural for me because init runs only once (like any other
>> init and like in 'for' or 'if')
>> */
>
> for(;;){int x=init;} // init runs infinitely many times
>
>
> if(auto a=init){} is really more trying to solve
>
> if(???){auto a=init;} // correct scoping but cannot refer to it in cond
>
> than
>
> {auto a=init; if(a){}} // correct scoping as long as there is no else
>
> That is why there is the language construct. That it has not been  
> adopted for while is because it is so much more useful for if, but I  
> personally think while could/should get the possibility as well.
>
>
>
>> auto obj = init;
>> while(obj) { work(obj); }
>>
>> This could confuse many people, I think. What do you think?
>>
>
> I think that by the same argument, you could say that the fact that the  
> body of a looping statement is executed multiple times could confuse  
> many people.
>
> 1 is more natural imho. (and 2 would be rather useless)

I disagree -- I think the semantics are confusing.

I'd say just leave the auto x = y for if and for loops, where there is a  
clear "run once" section.

Also, what do you do with a do while loop if the while contains this  
construct?

I just find the whole thing confusing.

-Steve