Should range foreach be iterating over an implicit copy?

[Ken]

On Friday, 18 May 2012 at 06:17:13 UTC, Lars T. Kyllingstad wrote:
> On Thursday, 17 May 2012 at 14:18:55 UTC, Steven Schveighoffer 
> wrote:
>> [...]  I believe someone has created a byRef struct that wraps 
>> a range and iterates it byRef (maybe dsimcha?)
>
> Nope, me. :)
>
> https://github.com/kyllingstad/ltk/blob/master/ltk/range.d#L12
>
> It only supports the input range primitives, because that was 
> all I needed when I wrote it.  I offered to contribute it to 
> Phobos (with support for the remaining range types, of course), 
> but the interest for it seemed rather low.
>
> -Lars

I've got a use for it right now.  I've implemented groupBy for my 
project, which is like std.algorithm.group except that instead of 
giving you the number of adjacent equivalent items, it gives you 
a lazy subrange of them.  So you end up with a range of subranges.

Trouble is, if you feed the inner range to foreach, it works on a 
copy of the inner range.  Which means that the outer range skips 
through the entire inner range you just iterated through in order 
to get to the next inner range.

Now if the range you feed to groupBy is a true non-ForwardRange, 
where copying it just creates an alias of some kind, this problem 
goes away.  Your wrapper looks like it would do the trick there.

At any rate, the whole thing makes an excellent use case for 
foreach(ref e; ref r).