Q on Patterns and Streams

[Tagir Valeev]

>   .mapMaybe(e -> Optional.ofNullable(e instanceof P(b) ? b : null) )

Btw it's already possible today because we have Stream.ofNullable!

.flatMap(e -> Stream.ofNullable( e instanceof P(b) ? b : null) )

On Wed, Jan 20, 2021 at 9:56 PM Brian Goetz <brian.goetz at oracle.com> wrote:

> Indeed, this is a question we've wrestled with.
>
> You are right that this is a sort of flatMap.  And that the current
> flatMap doesn't express it very nicely, or efficiently.
>
> We've recently added another form of flatMap, mapMulti, which is a lot
> more efficient; in that version, your example would be:
>
>      .mapMulti((e,s) -> { if (e instanceof P(b)) s.accept(b); })
>
> If `void` were a type, we could express this as an expression lambda:
>
>      .mapMulti((e,s) -> e instanceof P(b) ? s.accept(b) : void )
>
> A variant of mapMulti optimized for zero-or-one (which would be more
> sensible when Optional is a primitive class) might be:
>
>      .mapMaybe(e -> Optional.ofNullable(e instanceof P(b) ? b : null) )
>
> where mapMaybe takes a T -> Optional<U>.
>
> Note that the use of patterns here is limited to the case where you are
> going to package up all the bindings into a single value.
>
> There's some other possibilities but I want to think about them some
> more before discussing them.
>
>
> On 1/19/2021 7:30 PM, Johannes Kuhn wrote:
> > Turns out I sometimes write code that looks like this:
> >
> >     foos.stream().filter(e -> e instanceof Foo).map(e -> (Foo) e)...
> >
> > or
> >
> > foos.stream().filter(Foo.class::isInstance).map(Foo.class::cast)...
> >
> > Nicolas Fränkel has even written a blog post about this[1].
> >
> > Anyway, this can be expressed with a single stream operation:
> >
> >     foos.stream().flatMap(e -> e instanceof Foo foo ? Stream.of(foo) :
> > Stream.of())...
> >
> > I expect that form to work with any pattern in the future.
> > But it doesn't look great.
> >
> > Are there any plans around this?
> > Or is this something patterns shouldn't be used for?
> >
> > In a more general way - will there be a way to pass a pattern as an
> > argument to some other function?
> >
> > - Johannes
> >
> >
> >
> > [1]: https://blog.frankel.ch/changing-coding-habits-stream-friendly/
>
>