On Tuesday, 15 December 2015 at 17:36:52 UTC, Chris Wright wrote:
I noticed that some methods in Phobos will have very different
behavior with forward ranges that have reference semantics and
those that have value semantics.
Example:
auto range = getSomeRange();
auto count = range.walkLength;
foreach (element; range) { writeln(element); }
If getSomeRange returns a forward range that is a reference
type with no .length property, walkLength will exhaust the
range. The iteration after that will never enter the loop body.
However, if getSomeRange returns a range with value semantics
with no .length property, then iteration is still possible.
I haven't found documentation about how ranges are intended to
be used in D written by people who maintain Phobos. Is it
normal and expected that I should have to call .save
everywhere, manually? Was there discussion on whether this
should be the case or a published document containing the
reasoning behind the decision?
Unfortunately you must make no assumption about the ranges state
in generic code after calling a function that takes the range by
value. If you want to perform other operations after the call to
walklength, you'll need to have a new range created with save()
before that call.
The value vs reference rage creates challenges, but in practice
it will be something you occasionally hit.
I kind of feel like a range should always be taken by reference
so that it is predictable that the range will be modified if it
is value or reference semantics.
