Making sense of ranges

[Stewart Gordon]

On 24/03/2012 18:57, Ali Çehreli wrote:
<snip>
> Iterating an output range is also by popFront(). So what it says is, put this element to
> the output range and advance the range. There is a gotcha about this when the output range
> is a slice: Whatever is just put into the range is popped right away! :) [2]

I'm beginning to get it now: the purpose of an output range is to put new data into the 
underlying container.  So once you've put something in, the remaining range is what's left 
to be populated.  I had been thinking of outputting in terms of appending to the range, 
hence the confusion.

>> - Why is a range that can save checkpoints called a "forward" range?
>
> I agree. Here is my guess: The name of the ForwardRange comes from the fact that it is not
> double-ended. It can go only in one direction.
<snip>

Bidirectional ranges are forward ranges as well.

But I've just had a look at STL iterators, and it seems that the range category hierarchy 
has been lifted from that.  There, a "forward" iterator combines the functionality of an 
input iterator and an output iterator, hence it's your basic iterator that can move 
forwards and do what it likes with the data it walks over.  (It would seem that it's only 
an "output" iterator in that * returns an lvalue, which may or may not be actually 
assignable depending on the constancy of the element type.)

In D ranges, OTOH, the only thing that distinguishes a forward range from a general input 
range is the presence of a save method to make a copy of it.  Doesn't seem to have 
anything to do with either the "forward" name or the C++ origin thereof....

Something else I'm finding puzzling is moveFront, moveAt and moveBack.  Is D trying to be 
C++11 or something?  Move semantics don't seem to me to be useful in a language with GC.

Stewart.