If Statement with Declaration

[Jonathan Marler via Digitalmars-d]

On Wednesday, 19 July 2017 at 15:39:02 UTC, Steven Schveighoffer 
wrote:
> On 7/19/17 9:30 AM, sontung wrote:
>> So I was thinking of a way of extending if statements that 
>> have declarations. The following being as example of the 
>> current use of if statements with declarations:
>> 
>>      if(int* weDontPollute = someFunc())
>>      {
>>           // use weDontPollute
>>      }
>> 
>> That's great and all, but it only works by checking if the 
>> variable evaluates to true or false. Which is fine for a 
>> pointer but otherwise useless for anything else, like integers 
>> where zero is usually valid input (index for array). So 
>> currently the only way to do something like this in the 
>> language, that i've found, is to use a for statement.
>> 
>>      for(int i = someFunc(); i >= 0;)
>>      {
>>          // use i
>> 
>>          break;
>>      }
>> 
>> Not that ideal to use a for statement. It makes it hard to 
>> read and if the break condition isn't there it might very well 
>> be an infinite loop. So I was thinking of some sort of syntax 
>> like this:
>> 
>>      if(int i = someFunc(); i >= 0)
>>      {
>>          // use i
>>      }
>> Thoughts on this sort of feature?
>
>
> I really like the idea. Only thing I don't like is the 
> possibility for abuse/confusion/errors:
>
> if(int i = someFunc(); j >= 0) // typo, or weird relationship, 
> or just intentional obfuscation?
>
> It reminds me a bit of why we got rid of the comma operator.
>
> This is why I've liked suggestions in the past like:
>
> if((int i = foo()) >= 0)
>
> That is, you want to use 'if' on an expression while saving the 
> expression, but the if is only looking at a property of that 
> expression.
>
> Note this makes if(arr) (the correct meaning, that is ;) much 
> more palatable:
>
> if((auto x = getArray()).length)
>
> Don't get me wrong, if this syntax is what gets this idea in, 
> I'm fine with it.
>
> One possibility is to require usage of the declared variable in 
> the condition.
>
> -Steve

I respectfully disagree with this.  I recall many times where I 
want to declare variables that should be limited to the scope of 
the conditional block but aren't used in the condition itself, i.e

{
     auto a = ...;
     auto b = ...;
     while(something)
     {
         // use a and b
     }
}

I imagine this feature allowing it to be rewritten as:

while(auto a = ...;
       auto b = ...;
       something)
{
     // use a and b
}