Hi,
I recently came across the following code:
http://wiki.dlang.org/Higher_Order_Range_Pattern


I can't understand why the properties and methods of the structure are called in the correct order.
Why are the property `back()` and the method `popBack()` are not called even once?
In general, please explain how it all works.

```
import std.stdio, std.range;

struct Retro(Range)
{
    @property
    {
        auto ref front() { debug writeln("back"); return range_.back;  }
        auto ref back()  { debug writeln("front"); return range_.front; }
        bool empty()     { debug writeln("empty"); return range_.empty; }
    }

    void popFront() { debug writeln("popBack"); range_.popBack(); }
    void popBack()  { debug writeln("popFront"); range_.popFront(); }
	
    Range range_;
}

auto retro(Range)(Range range)
{
    return Retro!Range(range);
}

void main()
{
    import std.algorithm;
	
    auto arr = [1, 2, 3, 4, 5];
    assert(equal(retro(arr), [5, 4, 3, 2, 1]));
}
```
http://rextester.com/FMPGS76502

On 6/22/15 11:04 AM, Dennis Ritchie wrote:
> Hi,
> I recently came across the following code:
> http://wiki.dlang.org/Higher_Order_Range_Pattern
>
>
> I can't understand why the properties and methods of the structure are
> called in the correct order.
> Why are the property `back()` and the method `popBack()` are not called
> even once?

Because std.algorithm.equal does not use back or popBack.

> In general, please explain how it all works.

I have a feeling you are not understanding something. This code is pretty straightforward, I don't know what else to explain about it.

-Steve

On Monday, 22 June 2015 at 15:25:12 UTC, Steven Schveighoffer wrote:
> On 6/22/15 11:04 AM, Dennis Ritchie wrote:
>> Hi,
>> I recently came across the following code:
>> http://wiki.dlang.org/Higher_Order_Range_Pattern
>>
>>
>> I can't understand why the properties and methods of the structure are
>> called in the correct order.
>> Why are the property `back()` and the method `popBack()` are not called
>> even once?
>
> Because std.algorithm.equal does not use back or popBack.
>
>> In general, please explain how it all works.
>
> I have a feeling you are not understanding something. This code is pretty straightforward, I don't know what else to explain about it.
>
> -Steve

Thanks. I understand everything, just std.algorithm.equal introduced me to the great confusion :)
Now everything fell into place.