On Thursday, 19 September 2019 at 22:55:55 UTC, Jonathan M Davis
wrote:
For better or worse, ranges were more or less set up as a
linear hierarchy, and it's unlikely that use cases for
bidirectional ranges which aren't forward ranges are common. I
expect that it's a bit like infinite, bidirectional ranges. In
theory, they could be a thing, but the use cases for them are
uncommon enough that we don't really support them. Also, I
expect that most range-based algorithms which operate on
bidirectional ranges would require save anyway. A lot of
algorithms do to the point that basic input ranges can be
incredibly frustrating to deal with.
[ ... ]
Thanks for the characteristically thorough description of both
the design considerations and the history involved.
On reflection it occurs to me that the problem in my thinking may
be the idea that `save` should result in a full deep copy. If
instead we go by how `save` is implemented for dynamic arrays,
it's only ever a shallow copy: it's not possible to make valid
assumptions of reproducible behaviour if the original copy is
modified in any way.
If instead we assume that `save` is only suitable for temporary
shallow-copies that are made under the hood of algorithms, then
my problems go away.
Assuming we were redesigning the range API (which may happen if
we do indeed end up doing a Phobos v2), then maybe we could
make it so that bidirectional ranges don't have to be forward
ranges, but honestly _any_ ranges which aren't forward ranges
are a bit of a problem. We do need to support them on some
level for exactly the kind of reasons that you're looking to
avoid save with a bidirectional range, but the semantic
differences between what makes sense for a basic input range
and a forward range really aren't the same (in particular, it
works far better for basic input ranges to be reference types,
whereas it works better for forward ranges to be value types).
It occurs to me that the distinction we're missing here might
between "true" input ranges (i.e. which really come from IO of
some kind), which indeed must be reference types, versus "pure"
input ranges (which are deterministic, but which don't
necessarily allow algorithms to rely on the ability to save and
replay them).
As it stands, I don't think that we can change
isBidirectionalRange, because it's likely that most code using
it relies on its check for isForwardRange. So, I think that
we're stuck for the moment, but it is food for thought in a
possible range API redesign. I'll add it to my notes on the
topic. Some aspects of a range API redesign should look like
are pretty clear at this point, whereas others are very much an
open question.
Oh, I wasn't asking for any changes to the existing definition
(at least not without much thought from everyone!). I was just
wanting to understand the reasons for the current situation. But
thanks for putting it on the list of things to consider.
I may have some follow-up to your other remarks but I think at
least now I have a way forward with my code. Thanks!
