Proposal: delegates as aggregates in foreach statements

[James Dunne]

Chris Nicholson-Sauls wrote:
> James Dunne wrote:
> 
>> 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...
> 
> 
> True enough.  I didn't really mean anything scathing by it, anyhow.  
> Honestly, it was a bump message.  :)
> 

Seems to have worked then...

>> 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.
>>
> 
> [snip] Being able to just do (foreach (i, x; 
> &tree.walkInOrder) {...}) with .walkInOrder() being a simple method 
> would be, pardon the colloquialism, wicked sweet.
> 
> -- Chris Nicholson-Sauls

And how!

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James Dunne