On 10/7/10 8:24 CDT, Steven Schveighoffer wrote:
On Wed, 06 Oct 2010 19:09:15 -0400, Andrei Alexandrescu
<[email protected]> wrote:
On 10/6/10 12:50 CDT, Steven Schveighoffer wrote:
I agree all of this except for save() (not a secret to anyone who's read
my posts).
What was your alternative design that obviates save()?
First, let me discuss why I don't like save.
Let's classify ranges into two types. Primitive ranges are ones that
provide a range interface to data that otherwise does not have a range
interface. A good example is a container range.
Compound ranges are ranges that combine or alter range functionality
from a wrapped range or set of ranges. A good example is Zip or Chain.
Let's also define the trivial save implementation as a single line:
return this;
We can assume that compound ranges don't contain a trivial solution, but
only because the range(s) they contain could implement a non-trivial
solution. So we can qualify those trivial, even though they don't have
the canonical trival solution.
Most ranges that are not compound implement the trivial solution. This
includes all dcollections ranges and *ALL* non-compound ranges in phobos.
Then there are ranges which cannot implement save whatsoever, even if
they wanted to. A range that provides the range interface for a network
stream would be a good example.
That leaves us with ranges which cannot implement the trival solution,
but can implement save. A class-based range would be a good example.
Currently, there are zero examples of this type of range. That's right,
zero (grep 'save()' std/*.d). I contend that we have no place for such
ranges in D.
So my question is, what is the point of save? The whole point is for
this last class of ranges, so they can implement a way to copy the
iteration position in a way that isn't doable via simple assignment. But
there *AREN'T ANY* of these ranges in existence. Why do we have a
feature that is littered all over phobos and any range that wants to be
more than a basic imput range when the implementation is return this; ?
[snip]
I knew my retort for a while now, but neglected to write it down.
David Simcha added dynamic interfaces for std.range, in which save()
does work and is actually instrumental. That suggests save() is useful.
Now, dynamic interfaces are not just an obscure corner case, so I think
they're here to stay and provide a compelling use case.
Andrei
