Re: RFR[8238286]: 'Add new flatMap stream operation that is more amenable to pushing’

[Daniel Fuchs]

Hi Rémi,

On 25/06/2020 23:12, Remi Forax wrote:
>> if i have already have a BiConsumer<Consumer<Object>, Object>, i would like to
>> be able to call Stream<String>.mapMulti() with that bi-consumer as argument.
> and obviously, i got it wrong, Consumer<Object> is not a super-type of Consumer<String>, it should be a BiConsumer<Consumer<?>, Object> or a BiConsumer<? super Consumer<String>, Object>, etc.
> 

Right - I had actually to code it to convince me that

<R> StreamTest<R> mapMulti(BiConsumer<? super Consumer<R>, ? super T> 
mapper)

(where R is String) would actually accept a
BiConsumer<Consumer<?>, Object> or a
BiConsumer<Consumer<?>, Object> or a BiConsumer<? super 
Consumer<String>, Object>

However, if you try to implement such BiConsumers, and try to
implement their accept method to call consumer.accept(o); then
they fail to compile. Specifically:

     static BiConsumer<Consumer<?>, Object> c1 =
             new BiConsumer<Consumer<?>, Object>() {
         @Override
         public void accept(Consumer<?> consumer, Object o) {
             consumer.accept(o);
         }
     };

     static BiConsumer<Consumer<? super String >, Object> c2 =
             new BiConsumer<Consumer<? super String>, Object>() {
         @Override
         public void accept(Consumer<? super String> consumer, Object o) {
             consumer.accept(o);
         }
     };

will not compile:

StreamTest.java:12: error: incompatible types: Object cannot be 
converted to CAP#1
             consumer.accept(o);
                             ^
   where CAP#1 is a fresh type-variable:
     CAP#1 extends Object from capture of ?
/Users/danielfuchs/test/HttpRealTestAsync/src/StreamTest.java:20: error: 
incompatible types: Object cannot be converted to CAP#1
             consumer.accept(o);
                             ^
   where CAP#1 is a fresh type-variable:
     CAP#1 extends Object super: String from capture of ? super String

Which brings me back to my question: is there any value in having a
BiConsumer that is accepted by mapMulti, where the signature of the
BiConsumer doesn't allow it to act on its consumer argument without
casting?

best regards,

-- daniel