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Re: Lambda dereference

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From	Daniele Futtorovic <da.futt.news@laposte.net>
Newsgroups	comp.lang.java.programmer
Subject	Re: Lambda dereference
Date	Sat, 17 Mar 2018 03:10:21 +0100
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On 2018-03-17 00:42, Andreas Leitgeb wrote:
> Daniele Futtorovic <da.futt.news@laposte.net> wrote:
>> On 2018-03-16 19:26, Daniele Futtorovic wrote:
>>> interface Tagged {
>>>     boolean tag();
>>> }
>>> Stream<Tagged> stream = ...;
>>> You can do this (exhibit A):
>>>    Predicate<Tagged> p = Tagged::tag;
>>>    Collection<Tagged> untagged = stream.filter( p.negate()
>>> ).collect(Collectors.toSet());
>>> But you can't do this (exhibit B):
>>>    Collection<Tagged> untagged = stream.filter( (Tagged::tag).negate()
>>> ).collect(Collectors.toSet());
>>> Why not? It would be pretty neat. Or am I missing something?
>>
>> This works:
>> Collection<Tagged> untagged = stream.filter(
>> ((Predicate<Tagged>)Tagged::tag).negate() ).collect(Collectors.toSet());
>>
>> ... and it makes sense why it wouldn't accept it without the casting.
> 
> Huh, why?    Well, Java does have some "expressions" that only work as
> initializers (like braced list of elements for initializing arrays), but
> that wouldn't look to me like such a case - afterall one could use 
> Tagged::tag right there if it weren't for the ".negate()".
> 
> What would be the expression type of blank (Tagged::tag)?
> 

No, absolutely -- the compiler can't determine that Tagged::tag is a
Predicate without the context, and hence (Tagged::tag).negate() will
fail (parentheses just for clarification). I'd figured as much, but then
when I tried to add a cast, I didn't seem to be going anywhere. So I
thought, hey, that's something I could post to cljp! And when it was
posted, I thought, just a second, I'd better double-check some more.
And, oh fortuna, thou art a bitch, the cast worked.

Foot.values().forEach( Mouth.get()::insert ); :-/

-- 
DF.

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Thread

Lambda dereference Daniele Futtorovic <da.futt.news@laposte.net> - 2018-03-16 19:26 +0100
  Re: Lambda dereference Daniele Futtorovic <da.futt.news@laposte.net> - 2018-03-16 19:32 +0100
    Re: Lambda dereference Andreas Leitgeb <avl@logic.at> - 2018-03-16 23:42 +0000
      Re: Lambda dereference Daniele Futtorovic <da.futt.news@laposte.net> - 2018-03-17 03:10 +0100

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