Can't recreate a range?

Steven Schveighoffer schveiguy at
Thu Apr 30 19:11:09 UTC 2020

On 4/30/20 2:30 PM, 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!

There could be expensive operations done to construct the element that 
aren't necessary to iterate the range, but generally you achieve this by 
using something like map and cache.

But really the only valid use case for this is if you create a range but 
never use it. Some may need this, but I think a wrapper would be a 
better solution than enforcing a certain usage pattern.


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