[phobos] Would you find a ByRef range useful?

[Lars Tandle Kyllingstad]

On Thu, 2011-05-19 at 14:24 -0400, Robert Jacques wrote:
> On Thu, 19 May 2011 10:14:58 -0400, Lars Tandle Kyllingstad  
> <lars at kyllingen.net> wrote:
> > Below is a simple implementation of a range which iterates another
> > (input) range by reference, for those cases when using an
> > InputRangeObject is overkill.  I've used it quite a bit and I find it
> > immensely useful.
> >
> > It is sort of the opposite of save().  Where save() ensures that you are
> > only consuming a copy of the original range, byRef() ensures that you
> > are consuming the original range.
> >
> > Would this be useful for Phobos?  I guess it is a tad unsafe, since it
> > stores the address of the range, which may well be stored on the stack.
> >
> >
> > /** Range that iterates another range by reference. */
> > auto byRef(Range)(ref Range range) if (isInputRange!Range)
> > {
> >     static struct ByRef
> >     {
> >         private Range* _range;
> >
> >         static if (isInfinite!Range)
> >         {
> >             enum empty = false;
> >         }
> >         else
> >         {
> >             @property bool empty() { return (*_range).empty; }
> >         }
> >
> >         @property ElementType!Range front()
> >         {
> >             return (*_range).front;
> >         }
> >
> >         void popFront()
> >         {
> >             (*_range).popFront();
> >         }
> >     }
> >
> >     return ByRef(&range);
> > }
> >
> > unittest
> > {
> >     auto a = [1, 2, 3, 4];
> >     auto b = take(byRef(a), 2);
> >
> >     assert (equal(b, [1, 2]));
> >     assert (equal(a, [3, 4]));
> > }
> 
> I've wrote version of this for my Json library, so yes, I think it's  
> useful. I'd recommend a few things. First, if the Range is a reference  
> type, then you don't need the extra level of indirection. Second, you may  
> wish to include the rest of the range primitives, plus length/slicing, if  
> the base type supports them.

It definitely needs to be fleshed out a bit, yes.  Right now it's
sufficient for my uses, however, so I just thought I'd gauge the
interest before making any improvements.

I can't really see any elegant way to implement save() for such a range,
though, and without save() it doesn't qualify as a forward range, a
bidirectional range or a random-access range -- at least not as defined
by isForwardRange & co.  Is there then any point in implementing the
remaining primitives?

-Lars