Proposal: delegates as aggregates in foreach statements

[James Dunne]

Chris Nicholson-Sauls wrote:
> Bruno Medeiros wrote:
> 
>> Chris Nicholson-Sauls wrote:
>>
>>> An idea occurred to me last night, which I'm sure must have come up 
>>> before.  If it hasn't, I'm shocked, but I'm bringing it up (again?) 
>>> anyway.  Why not allow a delegate (or even function pointer?) to be 
>>> used as the "aggregate" parameter to a foreach statement, requiring 
>>> that it expose the same signature as a valid opApply method?  For 
>>> example:
>>>
>>> # class Foo {
>>> #   private int[] p_data;
>>> #
>>> #   int opApply (int delegate(inout size_t, inout int) dg) {
>>> #     int result = 0;
>>> #     foreach (inout i, inout x; p_data) {
>>> #       result = dg(i, x);
>>> #       if (result)
>>> #         break;
>>> #     }
>>> #     return result;
>>> #   }
>>> #
>>> #   int reverse (int delegate(inout size_t, inout int) dg) {
>>> #     int result = 0;
>>> #     foreach (inout i, inout x; p_data.reverse) {
>>> #       result = dg(i, x);
>>> #       if (result)
>>> #         break;
>>> #     }
>>> #     return result;
>>> #   }
>>> # }
>>> #
>>> # Foo foo = new Foo;
>>> # foreach (size_t i, int x; &foo.reverse)
>>> #   // ... do stuff ...
>>>
>>> One could even get real cute and use anonymous delegates:
>>>
>>> # foreach (size_t i, inout char[] x;
>>> #   delegate int(int delegate (inout size_t ii, inout char[] xx) {
>>> #     // implement iterator logic
>>> #   }
>>> # ) {
>>> #   // ... do stuff ...
>>> # }
>>>
>>> This, I think, would stand in the place of many uses of iterator 
>>> objects and mutators.
>>>
>>> -- Chris Nicholson-Sauls
>>
>>
>>
>> Hum, seems like a sound proposal. I think it could be good.
>>
>>
> 
> Well at least you thought so.  Doesn't look like it caught anyone else's 
> eye.
> 
> -- Chris Nicholson-Sauls

Just because nobody replies doesn't mean nobody reads.  There's a lot of 
content to plow through during the weekdays on this newsgroup.  I find 
myself guilty of "Mark Thread As Read" (Thunderbird) a lot recently for 
the threads I'm not interested in...

Anyway, about your proposal...

I don't much see much utility in *anonymous* delegates for iteration 
(lots of code duplication for commonly-iterated objects; and what's the 
point of including complex iterator code next to the body of the 
iteration itself?).

But, being able to select an iterator method for a class without 
breaking that method into its own iterator object does have its uses. 
For instance, reverse iteration and tree-walkers (pre-order, in-order, 
post-order, etc.) look to be good candidates here.

-- 
Regards,
James Dunne