Request to Include cats.data.Coproduct and cats.free.Inject

[raulraja]

The following PR adds cats.data.Coproduct and cats.free.Inject largely based on those found in Scalaz as in "Data types a la carte" (Swierstra 2008).

Coproduct & Inject can be used to compose Free based Algebras where an Application is nothing but the Coproduct resulting from the interaction of it's algebras.

In other words:

import cats.arrow.NaturalTransformation
import cats.data.{Xor, Coproduct}
import cats.free.{Inject, Free}
import cats.{Id, ~>}
import scala.collection.mutable.ListBuffer

/* Handles user interaction */
sealed trait Interact[A]
case class Ask(prompt: String) extends Interact[String]
case class Tell(msg: String) extends Interact[Unit]

/* Represents persistence operations */
sealed trait DataOp[A]
case class AddCat(a: String) extends DataOp[Unit]
case class GetAllCats() extends DataOp[List[String]]

type CatsApp[A] = Coproduct[DataOp, Interact, A]

def lift[F[_], G[_], A](fa: F[A])(implicit I: Inject[F, G]): Free[G, A] =
  Free.liftF(I.inj(fa))

class Interacts[F[_]](implicit I: Inject[Interact, F]) {
  def tell(msg: String): Free[F, Unit] = lift(Tell(msg))
  def ask(prompt: String): Free[F, String] = lift(Ask(prompt))
}

object Interacts {
  implicit def interacts[F[_]](implicit I: Inject[Interact, F]): Interacts[F] = new Interacts[F]
}

class DataSource[F[_]](implicit I: Inject[DataOp, F]) {
  def addCat(a: String): Free[F, Unit] = lift[DataOp, F, Unit](AddCat(a))
  def getAllCats: Free[F, List[String]] = lift[DataOp, F, List[String]](GetAllCats())
}

object DataSource {
  implicit def dataSource[F[_]](implicit I: Inject[DataOp, F]): DataSource[F] = new DataSource[F]
}

def program(implicit I : Interacts[CatsApp], D : DataSource[CatsApp]) = {

  import I._, D._

  for {
    cat <- ask("What's the kitty's name")
    _ <- addCat(cat)
    cats <- getAllCats
    _ <- tell(cats.toString)
  } yield ()
}

Interpreters omitted for simplicity but available in the free monads tutorial and Inject tests.

@non @ceedubs thanks for your consideration.

[non]

This looks like a great PR! I'm on a train so I can't do a detailed review yet, but on the surface it looks great. Thanks for updating the tutorial and including tests as well! 🐈

[adelbertc]

This is awesome, thanks! Looks like there is a legitimate build failure - I'm guessing Option#contains was added in 2.11 and missing from 2.10 :/

Other than that, lots of tests, lots of doc, I like it 👍

[ceedubs]

Minor: let's go ahead and make this class final to be consistent with other Ops classes.

[ceedubs]

Thanks a bunch @raulraja. I left some minor comments, but in general this is very clean, has good test coverage, and even comes with a great tutorial! 💯

[codecov-io]

Current coverage is 81.71%

Merging #671 into master will increase coverage by +0.74% as of 0ba18cd

@@            master    #671   diff @@
======================================
  Files          159     162     +3
  Stmts         2123    2198    +75
  Branches        72      72       
  Methods          0       0       
======================================
+ Hit           1719    1796    +77
  Partial          0       0       
+ Missed         404     402     -2

Review entire Coverage Diff as of 0ba18cd

Powered by Codecov. Updated on successful CI builds.

[raulraja]

@ceedubs @adelbertc @non addressed all your comments, let me know if anything else can be improved. thx!

[ceedubs]

Sorry for the minor details, but could you make the CodproductInstances* traits private[data]? That will make them consistent with the work done in #612.

[raulraja]

No worries, I want to get it right so I understand what the expectations are for future contributions.:) Added those access modifiers also on InjectInstances

[raulraja]

@ceedubs @adelbertc @non Added Free.inject[F[_], G[_]], NaturalTransformation.or with tests and CoflatMap and Foldable tests for Coproduct

[ceedubs]

Unfortunately, I think this will pick up the same instances as the .traverse check above. That's why I recommended the somewhat bizarre ListWrapper.foldable approach in another comment.

[raulraja]

I see, I'll give that a shot

[raulraja]

@ceedubs Would this do it? non@5ea5313

[ceedubs]

👍 💯

[non]

👍 Looks good to me!

[adelbertc]

👍 Thanks @raulraja , awesome PR