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

[Timon Gehr]

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)