On Sunday, 23 March 2014 at 00:50:34 UTC, Walter Bright wrote:
It's become clear to me that we've underspecified what an
InputRange is. The normal way to use it is:
while (!r.empty) {
auto e = r.front;
... do something with e ...
r.popFront();
}
no argument there. But there are two issues:
1. If you know the range is not empty, is it allowed to call
r.front without calling r.empty first?
If this is true, extra logic will need to be added to r.front
in many cases.
2. Can r.front be called n times in a row? I.e. is calling
front() destructive?
If true, this means that r.front will have to cache a copy in
many cases.
I think there is one design mistake with current InputRange rules
that makes usage so inconsistent. We have `empty` but don't have
any distinct `not yet started` state. If calling `popFront` at
least once was required before accessing `front`, I can't imagine
the case where having any non-trivial code in `front` would have
been necessary.
