RFC on range design for D2

[Andrei Alexandrescu]

Fawzi Mohamed wrote:
> I like the new proposal much more than the first.
> 
> I believe you will be able to use it successfully in std.algorithm.
> I still would have preferred an operation like a sameHead or 
> compareHeadPosition (that might or might not return the order, but at 
> least tests for equality) so that upon request (-debug flag?) one would 
> be able to make all range operation safe (with overhead) in a generic 
> way, but it is up to you.

Comparing for equality of heads is very important. For now you can 
obtain it as a non-primitive by invoking:

auto sameHead = r.before(s).done;

The above also show how "done" is not always very expressive. Also you 
can compare whether two ranges have the same end by invoking:

auto sameRange = r is s;

> I what I really care about is the following:
> I want foreach magic on all objects that support .done and .next, even 
> if they are not ranges.
> foreach is about iteration, iteration needs only .done and .next (a 
> generator, iterator whatever), and it should work with that.
> Do not force the range idea on foreach iteration.
> foreach is a language construct, not a library one and should allow for 
> maximum flexibility.

Yes. Walter asked me to send him the syntactic transformation that 
foreach and foreach_reverse need to do. Duck typing will be used so as 
long as you define the proper names you're in good shape.

> As extra nicety as each generator/iterator/range returns just one object 
> I would like to be able to do:
> 
> // i counts starting from 1, j iterates on iterJ and in parallel k 
> iterates on a.all
> foreach(i,j,k;1..$,iterJ,a.all){
>     //...
> }
> 
> and have it expanded to
> 
> Range!(int) r1=1..$;
> alias iterJ r2;
> typeof(a.all) r3=a.all;
> while(!(r1.done || r2.done || r3.done)){
>     typeof(r1.next) i=r1.next;
>     typeof(r2.next) j=r2.next;
>     typeof(r3.next) k=r3.next;
>     //...
> }

Walter and I were discussing about ranges exposing key, key1, ... keyn. 
In that case foreach with multiple arguments would work, and would bind 
each of the extra argument to key, key1 etc. respectively.


Andrei