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

A call to tell represents passing some typed data to the user. The specifics of exactly how this should be done depends upon the interpreter that is running the program and in particular how it chooses to handle that datatype.

If we call tell[A]("step-name", presentationData) we will get an F[Unit] emitted. presentationData must be of type A and A must be in the types supported by the interpreter.

"step-name" in this case is the step identifier - these are presented to the user but the exact form will depend upon the interpreter used. It may for example be used to construct messages telling the user which question they are answering, for forming URL’s or for persistence of data.

Step identifiers should be unique not only to all the tells in the journey but across all the asks and interacts too as otherwise the interpreters can get confused as they do not know the current place in the journey.

Type aliases

In our date-of-birth example we created a type alias called TellTypes which defines the types we are allowed to tell our user.

Given this TellTypes definition -

type TellTypes = Long :: NilTypes
type AskTypes = NilTypes

This code will fail to compile -

scala> def badType[F[_] : cats.Monad](
     |   interpreter: Language[F, TellTypes, AskTypes]
     | ): F[Unit] = {
     |   interpreter.tell("oh-no",
     | }
<console>:24: error: Cannot find java.time.LocalDate in TellTypes
Error occurred in an application involving default arguments.

The reason here is that we are attempting to present a value of type java.time.LocalDate to the user via a tell, but java.time.LocalDate is not in TellTypes, only Long is.

In order to get this to work we could modify our program to send the user a Long instead -

def goodType[F[_] : cats.Monad](
  interpreter: Language[F, TellTypes, AskTypes]
): F[Unit] = {

Or we could add a requirement that the interpreter must know how to present a java.time.LocalDate back to the user -

type TellTypes = java.time.LocalDate :: Long :: NilTypes

Because we do not ask the user for a Long in the above program we can remove it from TellTypes (though it being there will not prevent the program from compiling) -

type TellTypes = java.time.LocalDate :: NilTypes