[swift-users] 2 x lazy collection bugs

[Howard Lovatt]

Thanks for the reply Dmitri - comments inline below:

  -- Howard.

On 9 February 2016 at 16:17, Dmitri Gribenko <gribozavr at gmail.com> wrote:

> On Mon, Feb 8, 2016 at 8:56 PM, Howard Lovatt via swift-users
> <swift-users at swift.org> wrote:
> > Hi,
> >
> > I think I have found a couple of bugs in lazy collections; just checking
> > before filling. The following code is a filter map version of the Sieve
> of
> > Eratosthenes:
> >
> > func lazyFilterMapForEachLoop(limit: Int = 9) -> [Int] {
> >
> >     var possibles = Array(count: limit + 1, repeatedValue: true)
> >
> >     return (2 ... limit).lazy.filter { i in // Has to be lazy and
> sequential
> > so that `possibles` is updated before it is used
>
> Before looking at your questions below, I want to note that this is
> not a supported way of using the lazy subsystem.  The issue is that
> you seem to be relying on the evaluation order.  It is not guaranteed,
> even if you can reliably reproduce a certain evaluation order in the
> current version of the library.
>

Not sure where it says that it won't be sequential - do you have a
reference?

>
> > The filter closure is called twice per trial integer! First without
> > preceding to the map stage at all, i.e. all values are filtered out!
> Second
> > time through the numbers it does proceed to the map stage as you would
> > expect.
>
> Again, remember that you are using the *lazy* subsystem.  The
> evaluation order of the closures you are passing to the operations is
> not guaranteed.  Closures can be evaluated once, more than once, or
> not at all, and in the order that the library choses.  Closures are
> required to be pure, so you are not allowed to observe how many times
> and in which order they were actually evaluated.
>
> Not sure where it says it can be called more than one - the filter might
be a very expensive operation like a database lookup!


> The reason why the closure is evaluated twice is because you end up
> calling an eager map(), which tries to reserve the array capacity
> suitable for the result.  To do that, map() calls 'count' on the lazy
> filter collection, which triggers the computation the first time.
>
> map on a lazy collection results in another lazy collection, so I don't
see this.


> It is an open question though, whether calling the closure twice is
> profitable here, and what can we do about it.  The idea though is that
> those reallocations can sometimes be quite expensive when the closure
> is cheap to execute.  OTOH, when the closure is expensive to evaluate,
> calling it twice is a loss.
>
> > It produces an "Index out of range" error despite the fact that maximum
> > number processed is 9 which is an allowable index of `possibles`.
>
> Are you sure you didn't mean to use ..< instead of ...?
>

No I meant 2 ... limit, the array is of size limit + 1.

PS The equivalent code in Java works as expected.

>
> 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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