Is there a reason why popFront doesn't automatically return what front does?

If so I'm still missing a combined variant of pop and popFront in std.range.
Why isn't such a common operation in Phobos already?

On Saturday, 20 September 2014 at 18:59:03 UTC, Nordlöw wrote:
> Is there a reason why popFront doesn't automatically return what front does?
>
> If so I'm still missing a combined variant of pop and popFront in std.range.
> Why isn't such a common operation in Phobos already?

Sometimes after popping, the previous `front` is no longer valid, such as in the case of a buffer being reused. We should be careful about promoting using a previously read `front` after `popFront` until we figure out what we want to do about these "transient ranges".

If you want move semantics, use `moveFront`.

On Saturday, 20 September 2014 at 18:59:03 UTC, Nordlöw wrote:
> Is there a reason why popFront doesn't automatically return what front does?
>
> If so I'm still missing a combined variant of pop and popFront in std.range.
> Why isn't such a common operation in Phobos already?

So far I know isn't common use return value from popFront() at same time it's called. For example, checkout how is:

int[] a = [1,2,3];
foreach(int n; a) {}

is translated:

for(auto r = a; !r.empty; r.popFront())
{
  int n = r.front;
}

to return same as value in front by popFront() save previously value is needed:

int popFrontInt(out int[] arr)
{
   int current = arr[0]; // or arr.front
   arr = arr[1 .. $];
   return current;
}

(this isn't exactly like Phobos implementation and is int[]-only, btw)

the cost of the 'current' variable may be a bit expansive (one extra register use per function call) and useless, since it isn't used and a common use is one like the loop.

I think it's well-designed, IMHO...

> On Saturday, 20 September 2014 at 18:59:03 UTC, Nordlöw wrote:
> If you want move semantics, use `moveFront`.

Is this function part of phobos library? if so, where?

On Saturday, 20 September 2014 at 19:23:46 UTC, Jakob Ovrum wrote:
> Sometimes after popping, the previous `front` is no longer valid, such as in the case of a buffer being reused. We should be careful about promoting using a previously read `front` after `popFront` until we figure out what we want to do about these "transient ranges".
>
> If you want move semantics, use `moveFront`.

Excellent! Thanks!

On 09/20/2014 11:59 AM, "Nordlöw" wrote:
> Is there a reason why popFront doesn't automatically return what front
> does?
>
> If so I'm still missing a combined variant of pop and popFront in
> std.range.
> Why isn't such a common operation in Phobos already?

It is also related to exception safety. It took the C++ community to realize that a Stack data structure with pop() returning the top object cannot be made exception safe. The solution was to separate pop() from top().

Here is the original paper demonstrating the impossibility:


http://ptgmedia.pearsoncmg.com/imprint_downloads/informit/aw/meyerscddemo/DEMO/MAGAZINE/CA_FRAME.HTM

Here is Herb Sutter's presentation of the solution:

  http://www.gotw.ca/gotw/008.htm

Ali

On Saturday, 20 September 2014 at 19:23:46 UTC, Jakob Ovrum wrote:
> If you want move semantics, use `moveFront`.

But x.moveFront doesn't modify x.

What I want is to transform my uses of std.range from

if (!x.empty)
{
    x.front.doStuff;
    x.popFront;
}

into

if (!x.empty)
    if (auto front = x.stealFront)
    {
        front.doStuff;
    }

This is more functional/atomic, that is it reduces the risk of accidentally forgetting to call popFront at the end.

Destroy!

On Saturday, 1 November 2014 at 11:43:28 UTC, Nordlöw wrote:
> if (!x.empty)
>     if (auto front = x.stealFront)
>     {
>         front.doStuff;
>     }
>
> This is more functional/atomic, that is it reduces the risk of accidentally forgetting to call popFront at the end.

Forgot my explicit question:

So why isn't something like x.stealFront already in Phobos?

On Saturday, 20 September 2014 at 19:40:02 UTC, AsmMan wrote:
> Is this function part of phobos library? if so, where?

It's in std.range.

On Saturday, 1 November 2014 at 11:45:25 UTC, Nordlöw wrote:
> So why isn't something like x.stealFront already in Phobos?

First try here:

https://github.com/nordlow/justd/blob/master/range_ex.d#L14

Please comment!

On Saturday, 1 November 2014 at 13:22:34 UTC, Nordlöw wrote:
> https://github.com/nordlow/justd/blob/master/range_ex.d#L14
>
> Please comment!

What's the recommended way of making stealFront and stealBack inout here? Can I somehow use auto ref together with inout?