Contramaps a subscriber using a synchronous function.

This is useful in situations where you want to guarantee that messages that you produce always reach passed to the subscriber are always handled, but can't guarantee that the subscriber being contramapped will always handle them. For example, a subscriber that consumes Netty ByteBuf can't subscribe directly to an Pekko streams publisher since Pekko streams may drop the messages its publishing without giving any opportunity to release the ByteBuf, this can be used to to convert some other immutable message to a ByteBuf for consumption by the Netty subscriber.

Maps a publisher using a synchronous function.

This is useful in situations where you want to guarantee that messages produced by the publisher are always handled, but can't guarantee that the subscriber passed to it will always handle them. For example, a publisher that produces Netty ByteBuf can't be fed directly into an Pekko streams subscriber since Pekko streams may drop the message without giving any opportunity to release the ByteBuf, this can be used to consume the ByteBuf and then release it.

Does a map and contramap on the processor.