Background: how we got the generics we have (2020)(cr.openjdk.java.net)

    foo.doSomething()      // returns Iterable.
       .doSomethingElse(); // Iterable doesn't implement doSomethingElse();

    public <T, Z, F extends Function<? super T, Z>> Z apply(Function<F, Z> target, F fn) {
        return target.apply(fn);
    }

    @Test
    public void test() throws Exception {
        Stream<String> stream = Stream.of("foo", "bar", "baz");
        stream = apply(stream::flatMap, s -> Stream.of(s + "_a", s + "_b"));
        System.out.println(stream.collect(toList()));

        Optional<String> bean = Optional.of("bean");
        bean = apply(bean::flatMap, b -> Optional.of("soy"));
        System.out.println(bean);

        CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> "hello");
        CompletableFuture<String> future2 = apply(future1::thenCompose, string -> CompletableFuture.supplyAsync(() -> string + " world"));
        System.out.println(future2.get());
    }

    public interface FlatMappable<T> {
        <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper);
    }

    public class FlatMappableOption<T> implements FlatMappable<T> {
        private final Option<T> value;

        public FlatMappableOption<T>(Option<T> value) {
            this.value = value;
        }

        <U> Optional<U> flatMap(Function<? super T, ? extends Optional<? extends U>> mapper) {
            return value.flatMap(mapper);
        }
    }

    // similar wrappers for CompletionStage and Stream