Super-dee-duper D features

[Sean Kelly]

Andrei Alexandrescu (See Website For Email) wrote:
> Kirk McDonald wrote:
>> Sean Kelly wrote:
>>> Derek Parnell wrote:
>>>
>>>> On Tue, 13 Feb 2007 17:39:46 +0100, Frits van Bommel wrote:
>>>>
>>>>> Andrei Alexandrescu (See Website For Email) wrote:
>>>>>
>>>>>> Bill Baxter wrote:
>>>>>>
>>>>>>> Yeh, I don't get it either.  How would that help me implement 
>>>>>>> merge() from merge sort for instance?
>>>>>>
>>>>>> Merge bumps the iteration in both collections conditionally. The 
>>>>>> form above bumps the iteration in the two collections 
>>>>>> unconditionally, until one is finished; then it continues with the 
>>>>>> other until that is finished.
>>>>>
>>>>> In other words, it doesn't :(.
>>>>
>>>>
>>>> I imaging that the full syntax will also include this form ...
>>>>
>>>>   foreach (int x, i ; coll1) (int y, j ; coll2)   {
>>>>    ... use i and j ...
>>>>      if (somecondition)
>>>>         x = ...  // To set the index back or forward to some
>>>>                  // arbitary point in the array 'coll1'.
>>>>   }
>>>
>>>
>>> This currently works for built-in arrays but not for user-defined 
>>> types.  Also, I think the fact that it works as all is the result of 
>>> an implementation detail, not spec-defined behavior.
>>>
>>>
>>> Sean
>>
>> There's no reason a user-defined type couldn't implement this.
> 
> Exactly. Walter and I have bounced a couple of possibilities and 
> concluded that the feature is of "medium difficulty/medium usefulness". 
> Probably Walter will look into implementing this first:
> 
> foreach (i ; 0 .. n)
> {
>   ...
> }

Out of curiosity, how would these situations be handled:

   foreach (i ; n .. 0) {}         // A
   foreach_reverse (i ; 0 .. n) {} // B
   foreach_reverse (i ; n .. 0) {} // C

Sean