[swift-users] AnySequence and type erasure

[Svein Halvor Halvorsen]

Ok. Good to hear that I'm not too far off :)

Can you provide me with an example of a sequence type or two that has some
map or other function that would benefit from an optimized wrapper? Or:
under what circumstances would the stdlib implementation outperform mine?
I'm completely new to this all, and I'm trying to understand. Could some
random access collection type parallelize a filter? If you want I guarantee
correct ordering, under what circumstances does some sequence type work
better than mere iteration?


fre. 17. jun. 2016 kl. 15.49 skrev Dmitri Gribenko <gribozavr at gmail.com>:

> On Fri, Jun 17, 2016 at 1:39 AM, Svein Halvor Halvorsen via
> swift-users <swift-users at swift.org> wrote:
> > I'm sure people here know about the problem that AnySequence,
> AnyGenerator,
> > etc solves.
> > In Swift, a protocol can have an associated type, which is kinda like
> > generics, but you cannot declare a variable like this:
> >
> > let sequence: SequenceType<Int>
> >
> > If you want a sequence, any sequence, over ints you need to wrap the
> > protocol in a new concrete, generic type, AnySequence<T> that itself
> > implements the protocol SequenceType, and where the associated type
> Element
> > is equal to the generic type T.
> >
> > The standard library does this. And, like many others, I have tried to
> > implement this for my self, to try to better understand the problem, and
> I
> > expected that I would end up with a design very similar to what the
> standard
> > library does. However, I have not seen the need for the complex boxing
> > mechanism. I'm probably misunderstanding something, though.
> >
> > Can someone please look at this code, and give me constructive feedback?
> Is
> > this a novel and working solution, or am I missing something?:
>
> Hi,
>
> Your design should work, but it won't provide optimal performance.
> Sequence has a lot of requirements that can be customized by the
> conforming type (e.g., map()).  We need to forward those to the
> original type to get the best performance.
>
> While you can create a closure for each of the forwarded methods,
> let's say n in total, it is n times more wasteful compared to using
> just a single reference like the standard library does.
>
> Dmitri
>
> --
> main(i,j){for(i=2;;i++){for(j=2;j<i;j++){if(!(i%j)){j=0;break;}}if
> (j){printf("%d\n",i);}}} /*Dmitri Gribenko <gribozavr at gmail.com>*/
>
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