RFC on range design for D2

[Bill Baxter]

On Wed, Sep 10, 2008 at 4:52 PM, Don <nospam at nospam.com.au> wrote:
> Andrei Alexandrescu wrote:
>>
>> In most slice-based D programming, using bare pointers is not necessary.
>> Could then there be a way to use _only_ ranges and eliminate iterators
>> altogether? A container/range design would be much simpler than one also
>> exposing iterators.
>
>> http://ssli.ee.washington.edu/~aalexand/d/tmp/std_range.html
>
> I like this a lot. You've mentioned safety and simplicity, but it also seems
> to be a more powerful abstraction than STL-style iterators.
>
> Consider a depth-first-search over a tree. You have a start point, an end
> point, and some internal state (in this case, some kind of stack). The
> interesting thing is that the required internal state _may depend on the
> values of the start & end points_.

Or you can think of it as a current point and an stopping criterion.

> STL iterators don't model this very well, since they require a symmetry
> between iterators. Which creates the difficulty of where the internal state
> should be stored.

This is also why I argued in my other post on digitalmars.D that we
shouldn't be trying to force the start and end parts of a range to be
named with complete symmetry.  They have different purposes.  There
will generally be one current point that is relatively active and one
stopping criterion that is relatively fixed.

I would like to take back one thing, though.  In another post I said I
didn't think using * for getting the current value was a good idea
because it would be too hard to grep for.  I hadn't been considering
the RandomAccess ranges at that time.  I can't imagine *not* using
operators for the random access ranges -- it just makes too much
sense.  So if it's ok for random access then it should be ok for
forward and bidir to use operators too.   BUT, just as with the random
access ranges, I don't think there should be any synonyms.  Just use *
as the only way to get the current element of a range.

I think the desire to have a special "*" shortcut is as clear an
indication as any that Andrei in his heart of hearts agrees that the
two parts of a range are not really symmetric and should not be
treated as such.

--bb