front evaluated multiple time with joiner depending on where extra arg given

[Timothee Cour]

> In general, if you want to do something once per element which involves
> side
> effects, I would advise using foreach rather than trying to put it into a
> range.


using foreach breaks UFCS chains, and also ElementType != ForeachType


> But if you insist on doing so, the side effect should go in popFront,
> not front, or the side effects will often be happening more than once per
> element.
>

This won't work if side effect should occur _before_ element is popped. And
this is very awkward to use with map/reduce/filter, as it forces one to
write explicitly a range wrapper, defeating purpose of reusing phobos
components. Having to write a custom range (with pop/front/popBack etc)
just to support lambdas with side effects is a pain.

I'd like to propose a generic solution for that, see the email I just sent:

"std.range.cacheFront proposal&working code: wraps a range to enforce front
is called only once"
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