Almost finished the promise impl that implements correctly the monad and works in the same way as CompletableFuture with all tests passing \o/ I probably need to recheck all tests and write some additional ones but looks promising. And the additional, compile time flag promesa.impl.flatten variable is available for enabling flattering or not. The 11.x promesa probably will come with flattering disabled (the breaking change) with the ability to disable it with the flag.