is std.algorithm.joiner lazy?

[Jonathan M Davis via Digitalmars-d-learn]

On Thursday, April 07, 2016 08:47:15 Puming via Digitalmars-d-learn wrote:
> On Thursday, 7 April 2016 at 08:27:23 UTC, Edwin van Leeuwen
>
> wrote:
> > On Thursday, 7 April 2016 at 08:17:38 UTC, Puming wrote:
> >> On Thursday, 7 April 2016 at 08:07:12 UTC, Edwin van Leeuwen
> >> wrote:
> >>
> >> OK. Even if it consumes the first two elements, then why does
> >> it have to consume them AGAIN when actually used? If the
> >> function mkarray has side effects, it could lead to problems.
> >
> > After some testing it seems to get each element twice, calls
> > front on the MapResult twice, on each element. The first two
> > mkarray are both for first element, the second two for the
> > second. You can solve this by caching the front call with:
> >
> > xs.map!(x=>mkarray(x)).cache.joiner;
>
> Thanks! I added more elements to xs and checked that you are
> right.
>
> So EVERY element is accessed twice with joiner. Better add that
> to the docs, and note the use of cache.

I would note that in general, it's not uncommon for an algorithm to access
front multiple times. So, this really isn't a joiner-specific issue. If
anything, it's map that should get a note in its docs, not joiner. You
really should just expect front to be called multiple times. So, if that's a
problem, use cache. But joiner is not doing anything abnormal.

And it's not even the case that it necessarily makes sense to make a rule of
thumb that ranges should copy front instead of calling it multiple times,
because if front returns by ref, calling front multiple times is likely to
be cheapepr, and while we don't properly support non-copyable types (like
UniquePtr) with ranges right now, we really should, so if anything, it
becomes the case that algorithms should favor calling front multiple times
over copying its value.

So, there are pros and cons involved with copying front vs calling it
multiple times, and I think that both approaches are both pretty common at
this point. So, given how frequently it makes sense for map to allocate
(e.g. to!string(a)), map should probably have a note about cache, but
overall, it's just something that you need to be aware of. Regardless, I
don't think that it makes sense to put anything in joiner's docs about it.

- Jonathan M Davis