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

[Jonathan M Davis]

On Thursday, August 25, 2011 07:11:31 Steven Schveighoffer wrote:
> 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.

I don't see why it would be confusing. Because it's unclear whether it's set 
every time or whether it's a new variable every time? It's actually one of 
those things that I do periodically thinking that it works and then get 
frustrated when it doesn't, so I'd love to see it added.

- Jonathan M Davis