sequence_revisited.java

    public static <T> CompletableFuture<Stream<T>> sequence(Stream<CompletableFuture<T>> source) {
        return source
                .reduce(completedFuture(Stream.empty()),
                        (fl, fo) -> fl.thenCombine(fo, (s, o) -> concat(s, Stream.of(o))),
                        (fa, fb) -> fa.thenCombine(fb, Stream::concat));
    }