Gatherer API : wildcards complaint

[forax at univ-mlv.fr]

> From: "Viktor Klang" <viktor.klang at oracle.com>
> To: "Remi Forax" <forax at univ-mlv.fr>, "core-libs-dev"
> <core-libs-dev at openjdk.java.net>
> Sent: Wednesday, January 17, 2024 5:49:01 PM
> Subject: Re: Gatherer API : wildcards complaint

> Hi Rémi,

> Thank you for the feedback—examples would be much appreciated!

Here is an example with an interface and a class, 

interface Counter { 
void increment (); 
int value (); 
} 

Gatherer < String , Counter , Integer > count () { 
class CounterImpl implements Counter { 
int counter ; 

@Override 
public void increment () { 
counter ++; 
} 

@Override 
public int value () { 
return counter ; 
} 
} 
Supplier < CounterImpl > initializer = CounterImpl :: new ; 
Gatherer . Integrator < Counter , String , Gatherer . Downstream <? super Integer >> integrator = ( counter , _ , _ ) -> { 
counter .increment(); 
return true ; 
}; 
BiConsumer < Counter , Gatherer . Downstream <? super Integer >> finisher = ( counter , downstream ) -> { 
downstream .push( counter .value()); 
}; 
return Gatherer . ofSequential ( initializer , integrator , finisher ); // does not compile :( 
} 

void main () { 
System . out .println( Stream . of ( "foo" ).gather(count()).findFirst().orElseThrow()); 
} 

if instead of explicitly typing each functions, we directly call ofSequential, it works 

return Gatherer . ofSequential ( 
CounterImpl :: new , 
( Counter counter , String _ , Gatherer . Downstream <? super Integer > _ ) -> { 
counter .increment(); 
return true ; 
}, 
( Counter counter , Gatherer . Downstream <? super Integer > downstream ) -> { 
downstream .push( counter .value()); 
} 
); 

because here, CounterImpl::new is inferred as Supplier<Counter>. 

> Cheers,
> √

> Viktor Klang
> Software Architect, Java Platform Group
> Oracle

> From: core-libs-dev <core-libs-dev-retn at openjdk.org> on behalf of Remi Forax
> <forax at univ-mlv.fr>
> Sent: Wednesday, 17 January 2024 16:55
> To: core-libs-dev <core-libs-dev at openjdk.java.net>
> Subject: Gatherer API : wildcards complaint
> Hello,
> this is a minor complaint but I do not see a raison to not getting this right.

> Currently the Gatherer API does not use the wildcard correctly, which is not
> fully an issue because there is "enough" wildcards that if you rely on the
> inference, it will work.

> The problem is that when you write code, you make mistakes and usually when you
> have a typing issue, a way to debug it is to fix the type arguments
> de-activating the inference.
> But because there are some missing wildcards, this debugging strategy just fail
> flat with more typing errors.

> I think that fixing the missing wildcards will hep users (or a least me) to have
> a better experience when using the Gatherer API.

> I can help and propose a PR if you want.

> regards,
> Rémi
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