I have always treated ranges with this expectation:
1. front gets you the current element of the range. Calling front
multiple times without calling popFront should get you the same value.
2. popFront moves you to the next element of the range if it exists.
However, there are some ranges which can violate this. For example map:
int i = 0;
auto r = iota(int.max).map!((a) => ++i);
auto e1 = r.front;
auto e2 = r.front;
assert(e1 + 1 == e2);
But I consider this a programming error. It's acceptable to me that it's
possible to write bugs using ranges, no programming language is immune
to that.
However, Joseph Wakeling pointed out[1] the generate range[2]:
auto r = generate(() => ++i);
// same result
But unlike map, there isn't a way to make generate work as a proper
range, because as Joe pointed out, popFront does nothing, front does all
the work that popFront would do.
There was a bit of discussion when this function was being vetted[3],
and IMO, this did not result in the correct function.
For example, generate.popFrontN does nothing. generate.drop does
nothing. And of course, calling front multiple times does not yield the
same answer, unless you provide a lambda that does the same thing every
time (a useless case).
The counter-argument seems to be that if you cache the front element,
then then making a copy of the range via take can repeat the cached
element[4]. I find this argument severely lacking -- non-forward ranges
are not meant to be copied and expected to operate properly, it's why we
require calling save.
I'd say violating the expectations of what popFront and front do is more
egregious than a particular use case, no matter how valid that case is.
I'd like to fix this bug, but I see there were quite a few major D
contributors in the camp that wanted this function to behave the way it
is. So I'd rather first debate the merits here.
-Steve
[1] https://forum.dlang.org/post/ttyzjfmegkmbluumi...@forum.dlang.org
[2] http://dlang.org/phobos/std_range.html#.generate
[3] https://github.com/dlang/phobos/pull/2606
[4] https://github.com/dlang/phobos/pull/2606#issuecomment-58740715
