Feature request: extending comma operator's functionality

[Don Clugston]

On 05/10/12 15:35, monarch_dodra wrote:
> On Friday, 5 October 2012 at 00:22:04 UTC, Jonathan M Davis wrote:
>> On Friday, October 05, 2012 02:08:14 bearophile wrote:
>>> [SNIP]
>>> Regarding definition of variables in D language constructs, there
>>> is one situation where sometimes I find D not handy. This code
>>> can't work:
>>>
>>> do {
>>> const x = ...;
>>> } while (predicate(x));
>>>
>>>
>>> You need to use:
>>>
>>> T x;
>>> do {
>>> x = ...;
>>> } while (predicate(x));
>>
>> Yeah. That comes from C/C++ (and is the same in Java and C#, I
>> believe). I
>> don't know why it works that way. It's definitely annoying.
>>
>> [SNIP]
>>
>> - Jonathan M Davis
>
> Because it's the only way to guarantee that x exits when you reach the
> end of the loop.
>
> do {
>    if(true) continue; //Yawn... skip.
>    const x = ... ;
> } while (predicate(x)); //What's x?
>
> Basic goto limitations. Unlike goto though, inserting a "continue"
> should never create a compile error, so the compiler *has* to guarantee
> that the if condition references nothing inside its own block.
>
> It is annoying, but nothing that can't be fixed with a scope bloc.

My feeling is that  do{}while() is a fairly useless concept, and this is 
part of the reason.
In my experience genuine do-while loops are extremely rare, and it only 
takes a slight change to the loop to force a different structure to be used.
Conditional loops which don't follow the while(){...} pattern normally 
follow the loop-and-a-half pattern, also known as begin-while-repeat (I 
think that's the name Knuth used). I'll call it 'super do':

super do {
    foo();
    while(cond);
    bar();
}

which in D is better modelled by:

for (;;)
{
    foo();
    if (!cond) break;
    bar();
}

rather than by a do-while loop. But it's a bit ugly, and doesn't enforce 
a single break. IMHO it's a shame we've gone with the fairly useless 
do-while, rather than cleaning up the syntax for loop-and-a-half.

I don't think the proposed changes bring us much closer to a useful 
language construct.