javaadvent_sample4.java

public static class CompositionSources<B>
{
    private CompositionSources(){ }

    public interface Partial<B>
    {
        CompletableFuture<Outcome<B>> apply(CompletableFuture<Outcome<B>> b);
    }

    public interface MergingStage<B, V>{
        Partial<B> by(BiFunction<Outcome<B>, Outcome<V>, Outcome<B>> f);
    }

    public <V> MergingStage<B, V> value(CompletableFuture<Outcome<V>> value){

        return f -> builder
                 -> builder.thenCombine(value, (b, v) -> f.apply(b, v)
                                                          .dependingOn(b)
                                                          .dependingOn(v));
    }

    public static <B> CompositionSources<B> stickedTo(Class<B> clazz)
    {
        return new CompositionSources<>();
    }
}