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Lambda dereference

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  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

#38187 — Lambda dereference

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?

-- 
DF.

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#38188

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?
> 

Nevermind, I'm an idiot. I had tried to cast it, and didn't seem to
manage, but must have screwed up somewhere.

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.

-- 
DF.

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#38189

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)?

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#38190

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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