def interact[Tell, Ask](stepId: String, value: Tell): F[Ask]

interact is simply the combination of an ask and a tell together.

It has a type parameter for both the datatype being presented to the user (Tell) and being asked of the user (Ask). A confirmation page presenting a user with an address and asking them for a Yes/No answer could be modeled as interact[Address, Boolean]("confirm-address", addr).

To be precise both ask and tell are defined in terms of interact -

def tell[A](stepId: String, value: A): F[Unit] = 
  interact[A, Unit](stepId, value)

def ask[A](stepId: String): F[A] = 
  interact[Unit, A](stepId, ())