An example of substituability test that is recursive

[forax at univ-mlv.fr]

----- Mail original -----
> De: "Brian Goetz" <brian.goetz at oracle.com>
> À: "Remi Forax" <forax at univ-mlv.fr>
> Cc: "Karen Kinnear" <karen.kinnear at oracle.com>, "valhalla-spec-experts" <valhalla-spec-experts at openjdk.java.net>
> Envoyé: Jeudi 31 Janvier 2019 17:56:47
> Objet: Re: An example of substituability test that is recursive

> Currently, `==` is almost useless on lambdas, as we disclaim nearly all
> promises.  What this would mean is that `==` becomes slightly less useless and
> slightly more expensive.  It’s not obvious this is a bad trade (or that it
> really matters, because people are discouraged from using `==` on lambdas
> anyway.)
> 

I agree.
and it's still useless because there is no guarantee that with
  Runnable r1 = () -> {};
  Runnable r2 = () -> {};
r1 and r2 use the same proxy, so r1 == r2 can still return false.


I'm just saying that having recursive value types are more frequent that what i was thinking before.

Rémi


>> On Jan 31, 2019, at 11:53 AM, Remi Forax <forax at univ-mlv.fr> wrote:
>> 
>> Thinking a little more about this example,
>> i think it will be more common if we retrofit lambdas to be value type because a
>> series of composition of lambdas is a kind of linked list in term of data
>> structure in memory.
>> 
>> For the composition of lambdas, a stack overflow is unlikely because otherwise
>> calling the lambda will stack overflow too but it means that == will be slow
>> (because it does a recursive comparison).
>> 
>> Rémi
>> 
>> ----- Mail original -----
>>> De: "Remi Forax" <forax at univ-mlv.fr>
>>> À: "Karen Kinnear" <karen.kinnear at oracle.com>
>>> Cc: "valhalla-spec-experts" <valhalla-spec-experts at openjdk.java.net>
>>> Envoyé: Jeudi 31 Janvier 2019 12:19:32
>>> Objet: An example of substituability test that is recursive
>> 
>>> Hi Karen,
>>> here is an example that recurse to its death with the current prototype
>>> 
>>> import java.lang.invoke.ValueBootstrapMethods;
>>> import java.util.stream.IntStream;
>>> 
>>> public class Substituable {
>>> static value class Link {
>>>   private final int value;
>>>   private final Object next;
>>> 
>>>   public Link(int value, Object next) {
>>>     this.value = value;
>>>     this.next = next;
>>>   }
>>> 
>>>   static Object times(int count) {
>>>     return IntStream.range(0, count).boxed().reduce(null, (acc, index) -> new
>>>     Link(index, acc), (l1, l2) -> { throw null; });
>>>   }
>>> }
>>> 
>>> 
>>> public static void main(String[] args) {
>>>   var l = Link.times(1_000);
>>> 
>>>   //System.out.println(l == l);
>>>   System.out.println(ValueBootstrapMethods.isSubstitutable(l, l));
>>> }
>>> }
>>> 
>>> 
> >> Rémi