Why do you need that?

I am in a case where I can optimize the consumer side depending if it's a single value or N

and I know for certain single value will always from from a promise-chan

As I understand it, you pass a promise-chan only when the consumer already expects to consume just once. May I ask what’s your case in more detail?

no not really. Think setting up http content-length early (upon sending headers) for single shot response vs chunked header for N messages). I could set chunked for all and be done with it. but it is problematic to do that for a few reasons and I'd rather avoid it.

sending headers and writing the actual body of the response are done separately (working with vertx http client in that case)

I think I might just make this stuff explicit I think instead of relying on shape of input