Add Predicate.of(), Consumer.of(), etc.

[Paul Sandoz]

On May 6, 2015, at 1:58 PM, Attila Szegedi <attila.szegedi at oracle.com> wrote:

> On May 6, 2015, at 12:37 PM, Paul Sandoz <paul.sandoz at oracle.com> wrote:
>> 
>> 
>> On May 2, 2015, at 11:31 PM, Remi Forax <forax at univ-mlv.fr> wrote:
>> 
>>> Hi all,
>>> today, I stubble on a variant of JDK-8050818 [1],
>>> trying to call negate() on a lambda which is not yet a Predicate (due to target typing) which requires either to cast the lambda to a Predicate and everybody knows that cast are evil or to add a local variable.
>>> 
>>> I think there is a way to fix that, it's not very orthodox so I let you judge.
>>> The idea is to introduce a static method 'of' on Predicate,
>>> class Predicate<T> {
>>>  ...
>>>  public static <T> Predicate<T> of(Predicate<T> predicate) {
>>>    return predicate;
>>>  }
>>> }
>>> 
>>> so one can write:
>>> stream.filter(Predicate.of(String::isEmpty).negate())
>>> compared to
>>> stream.filter(((Predicate<String>)String::isEmpty).negate())
>>> 
>>> so the question is, does it it worth the cost to introduce a static method that basically do nothing on all functional interfaces of java.util.function.
>>> 
>> 
>> That does have the virtue of not adding a static method per operation but i still cannot bring myself to add such methods as a work around for seems like a compiler inference problem (albeit one in which might be very tricky or not possible to solve).
> 
> I think it is firmly in the category of “very tricky”, but probably still possible, albeit (from my point of view) undesirable.
> 
> String::isEmpty will not, on its own, have a method named .negate(); the compiler can’t really infer the programmer’s intent to make it into a Predicate, not without a fairly high level reasoning (filter needs a Predicate; should the compiler perform an exhaustive search of the visible functional interfaces to see which one has a method with signature “Predicate negate()”?). So, yeah, it seems like in this case the programmer needs to communicate the intent. 
> 
> I think inference would be *possible*, but it’d be expensive, and would still allow for either ambiguities or accidentally matching something unexpected, so I think it would also be undesirable.
> 

Ok.


> I don’t have an opinion of whether we want an “of” static method on Predicate, as then it would have to indeed be introduced on many other interfaces. It’s more appealing than a cast, certainly; especially since the cast apparently needs to specify the explicit type parameter too.
> 

I guess it's too late to consider an implicitly declared method along the lines of what Remi suggests. FWIW i always found such methods on enum a little too magical. And perhaps it's too confusing to consider an invocation mode where "this" can be an implicit first parameter.


>> 
>> In some respects i wonder if the default methods on the functional interfaces are an attractive nuisance.
> 
> Meaning, if .negate(Predicate) were a static method on the Predicate class instead of a default method, then stream.filter(Predicate.negate(String::isEmpty)) would be possible? Yeah…
> 

Yeah. We are now in the unfortunate position where to alleviate this problem we might require duplicate static and default methods. I have been sitting on the issue a bit and nearly closed it a few times :-)

Paul.