foreach, an analogy

[Reiner Pope]

Walter Bright wrote:
> Bruno Medeiros wrote:
>> As of DMD now, the only advantage in 'foreach_reverse' is ephemerous: 
>> it allows efficient reverse iteration of arrays.
> 
> That's the most important use case.
> 
>> But couldn't the compiler easily detect this:
>>   foreach(Foo f; &fooarray.opApplyReverse) { ...
>> and and compile it as if it were a:
>>   foreach_reverse(Foo f; fooarray) { ...
> 
> Yes, it could. But it looks like a hack. A little syntactic sugar makes 
> a big difference.

Here's a better solution: allow trailing delegates, and define iteration 
according to them, making user iteration first-class. Then, mandate that 
arrays support forwards and backwards iteration using, eg, 'each' and 
'each_r' and the compiler can optimize such calls away:

  [0,1,2].each (i) writefln(i); // Prints 0 1 2
  [0,1,2].each_r (i) writefln(i); // Prints 2 1 0

void random(int[] array, void print(int i, out IterationStatus v), inout 
IterationStatus s) { /* define my own random iterator */ }

  [0,1,2].random (i) writefln(i); // Prints 0 2 1

You get the efficiency and an even nicer syntax.

Cheers,

Reiner