Modeling Video Store with Free

Using Freasy and cats Free Monads to model a domain.

Example

Freasy macro allows for very concise code:

1. Define DSL:

   @free trait VideoRental {
     sealed trait DSL[A]
     type VideoRentalF[A] = Free[DSL, A]
     def addInventory(movie: Movie, qty: Int): VideoRentalF[Set[DVD]]
     def searchForDVD(movie: Movie): VideoRentalF[Option[DVD]]
     def rentDVD(dvd: DVD): VideoRentalF[Unit]
     def returnDVD(dvd: DVD): VideoRentalF[Unit]
   }

2. Implement service:

   override def interpreter() = new VideoRental.Interp[ErrorOr] {
     override def addInventory(movie: Movie, qty: Int): ErrorOr[Set[DVD]] = ...
     override def searchForDVD(movie: Movie): ErrorOr[Option[DVD]] = ...
     override def rentDVD(dvd: DVD): ErrorOr[Unit] = ...
     override def returnDVD(dvd: DVD): ErrorOr[Unit] = ...
   }

Using Freek

Combine two different Free algebras (as long as they use same return type):

type PRG = Logging.DSL :|: VideoRental.DSL :|: NilDSL
val PRG = DSL.Make[PRG]

val VS = VideoRental.DSL
val LOG = Logging.DSL

// Create program
val program = for {
    _     <- LOG.Info(s"Adding $movie").freek[PRG]
    dvds  <- VS.AddInventory(movie, qty).freek[PRG]
    dvd   = dvds.head
    _     <- LOG.Info(s"Renting $dvd").freek[PRG]
    _     <- VS.RentDVD(dvd).freek[PRG]
    _     <- LOG.Info(s"Returning $dvd").freek[PRG]
    res   <- VS.ReturnDVD(dvd).freek[PRG]
} yield res

// Combine interpreters for VideoRental and Logging
val combinedInterpreter: Interpreter[PRG.Cop, ErrorOr] = StdoutLogging().interpreter :&: InMemory().interpreter

val result = program.interpret(combinedInterpreter)