Scala provides a filterNot method as a work around. This is a less generic fix,
but arguably more pragmatic for the common case of the streams API.
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From:"Attila Szegedi" <attila.szeg...@oracle.com>
Date:Wed, May 6, 2015 at 4:59 am
Subject:Re: Add Predicate.of(), Consumer.of(), etc.
On May 6, 2015, at 12:37 PM, Paul Sandoz <paul.san...@oracle.com> wrote:
>
>
> On May 2, 2015, at 11:31 PM, Remi Forax <fo...@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.
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.
>
> 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…
Attila.
>
> Paul.
>
