As it is implemented right now, yes, otherwise the hashtbl above (https://github.com/Incubaid/arakoon/pull/90/files#L26R27) has an 'a in its type which the compiler complains about.
So for now only detached computations returning strings are supported. I think each return type needs it's own Hashtbl tracking the openstanding threads? Or maybe some other clever implementation :), will think if I can find a better way.