Consevutive calls to r.front

[Jonathan M Davis via Digitalmars-d-learn]

On Monday, June 08, 2015 00:42:07 Mike Parker via Digitalmars-d-learn wrote:
> When implementing a custom range, is it correct to say that
> consecutive calls to r.front with no intervening calls to
> popFront should return the same value? Seems like I read
> something along those lines before, but I can't find it anywhere.

Yes. I was actually talking with Walter about this at dconf. Two consecutive
calls to front must return equivalent values (but not necessarily exactly
the same object - e.g. map!((a) => to!string(a))(range) is going to return
equal strings, but they won't be the exact same string). It would be a
fundamental violation of the range concept for multiple calls to front
to return values that were not equal if popFront were not called in between.

Also, a range cannot depend on empty or front being called in order to
iterate correctly. It _can_ choose to do work in those functions instead of
just popFront (e.g. map does), but it has to work to just have popFront
called without calling empty or front (e.g. if you know that the range has
more than 10 elements, you should be free to call popFront 10 times without
checking empty or accessing front).

- Jonathan M Davis