On Thursday, 30 April 2020 at 18:30:14 UTC, H. S. Teoh wrote:
On Thu, Apr 30, 2020 at 06:05:55PM +0000, Paul Backus via
Digitalmars-d-learn wrote: [...]
Doing work in popFront instead of front is usually an
anti-pattern, since it forces eager evaluation of the next
element even when that element is never used. You should only
do this if there's no reasonable way to avoid it.
Really?? IME, usually you *need* to do work in popFront instead
of front, because otherwise .empty wouldn't be able to tell
whether there is another element or not. E.g., in filtering a
range based on some criterion on each element, you can't defer
computing the next element until .front because you can't
predict whether there will be another element that won't be
dropped by popFront.
Also, for ranges based on generator functions, if .front is
lazy then you need to keep extra baggage around your range to
indicate whether or not the generator has been invoked yet;
it's easier to just always compute the next element eagerly and
cache it, and .front just returns the cached data.
Even when the range involves some expensive per-element
computation, I find that it's simpler to just compute and cache
in .popFront instead of adding extra baggage to .front to know
when the computation has already been performed.
I'm hard-pressed to come up with an example where deferring
computation to .front is a good idea!
T
Well, I just remembered that I had a tab open to this:
http://ddili.org/ders/d.en/ranges.html
Reading, you see the following:
* empty: specifies whether the range is empty; it must return
true when the range is considered to be empty, and false otherwise
* front: provides access to the element at the beginning of the
range
* popFront(): shortens the range from the beginning by removing
the first element
Looking at that, if popFront shortens the range, then to me it
sounds like the work should be done in popFront.
