Cats-tagless is a small library built to facilitate transforming and composing tagless final encoded algebras.

Installation

Cats-tagless is currently available for Scala 2.12 and 2.13 and Scala.js.

Add the following settings in build.sbt

libraryDependencies +=
  "org.typelevel" %% "cats-tagless-macros" % latestVersion // latest version indicated in the badge above

Compile / scalacOptions ++= {
  CrossVersion.partialVersion(scalaVersion.value) match {
    case Some((2, n)) if n >= 13 => "-Ymacro-annotations" :: Nil
    case _ => Nil
  }
}

libraryDependencies ++= {
  CrossVersion.partialVersion(scalaVersion.value) match {
    case Some((2, n)) if n >= 13 => Nil
    case _ => compilerPlugin("org.scalamacros" % "paradise" % "2.1.1" cross CrossVersion.full) :: Nil
  }
}

Auto-transforming tagless final interpreters

Say we have a typical tagless encoded algebra ExpressionAlg[F[_]]

import cats.tagless._

@autoFunctorK
trait ExpressionAlg[F[_]] {
  def num(i: String): F[Float]
  def divide(dividend: Float, divisor: Float): F[Float]
}

With Cats-tagless you can transform this interpreter using Cats' FunctionK, i.e, you can transform an ExpressionAlg[F] to an ExpressionAlg[G] using a FunctionK[F, G], a.k.a. F ~> G. Cats-tagless generates a FunctorK instance for your algebra.

The @autoFunctorK annotation adds the following line (among some other code) in the companion object.

object ExpressionAlg {
  implicit def functorKForExpressionAlg: FunctorK[ExpressionAlg] =
      Derive.functorK[ExpressionAlg]
}

This functorKForExpressionAlg is a FunctorK instance for ExpressionAlg which can map a ExpressionAlg[F] to a ExpressionAlg[G] using a FunctionK[F, G].

Note that the usage of @autoFunctorK, like all other @autoXXXX annotations provided by cats-tagless, is optional, you can manually add this instance yourself.

For example, if you have an interpreter of ExpressionAlg[Try]

import util.Try

object tryExpression extends ExpressionAlg[Try] {
  def num(i: String) = Try(i.toFloat)
  def divide(dividend: Float, divisor: Float) = Try(dividend / divisor)
}

You can transform it to an interpreter of ExpressionAlg[Option]

import cats.tagless.implicits._
import cats.implicits._
import cats._

val fk : Try ~> Option = λ[Try ~> Option](_.toOption)

tryExpression.mapK(fk)
// res0: ExpressionAlg[Option]

Note that the Try ~> Option is implemented using kind projector's polymorphic lambda syntax.

Obviously, FunctorK instance is only possible when the effect type F[_] appears only in the covariant position (i.e. the return types). For algebras with effect type also appearing in the contravariant position (i.e. argument types), Cats-tagless provides a InvariantK type class and an autoInvariantK annotation to automatically generate instances.

@autoFunctorK also add an auto implicit derivation, so that if you have an implicit ExpressionAlg[F] and an implicit F ~> G, you can automatically have a ExpressionAlg[G]. It works like this

import ExpressionAlg.autoDerive._

implicitly[ExpressionAlg[Option]]  //implicitly derived from a `ExpressionAlg[Try]` and a `Try ~> Option`

This auto derivation can be turned off using an annotation argument: @autoFunctorK(autoDerivation = false).

Example: stack safety Free

With Cats-tagless, you can lift your algebra interpreters to use Free to achieve stack safety.

For example, say you have an interpreter using Try

@finalAlg @autoFunctorK
trait Increment[F[_]] {
  def plusOne(i: Int): F[Int]
}

implicit object incTry extends Increment[Try] {
  def plusOne(i: Int) = Try(i + 1)
}

def program[F[_]: Monad: Increment](i: Int): F[Int] = for {
  j <- Increment[F].plusOne(i)
  z <- if (j < 10000) program[F](j) else Monad[F].pure(j)
} yield z

Obviously, this program is not stack safe.

program[Try](0)
//throws java.lang.StackOverflowError

Now, let's use auto derivation to lift the interpreter with Try into an interpreter with Free

import cats.free.Free
import cats.arrow.FunctionK
import Increment.autoDerive._

implicit def toFree[F[_]]: F ~> Free[F, *] = λ[F ~> Free[F, *]](t => Free.liftF(t))

program[Free[Try, *]](0).foldMap(FunctionK.id)
// res9: scala.util.Try[Int] = Success(10000)

Again, the magic here is that Cats-tagless auto derive an Increment[Free[Try, *]] when there is an implicit Try ~> Free[Try, *] and a Increment[Try] in scope. This auto derivation can be turned off using an annotation argument: @autoFunctorK(autoDerivation = false).

Horizontal composition with @autoSemigroupalK

You can use the SemigroupalK type class to create a new interpreter that runs both interpreters and return the result as a cats.Tuple2K. The @autoSemigroupalK attribute adds an instance of SemigroupalK to the companion object. Example:

@autoSemigroupalK
trait ExpressionAlg[F[_]] {
  def num(i: String): F[Float]
  def divide(dividend: Float, divisor: Float): F[Float]
}


val prod = tryExpression.productK(optionExpression)
prod.num("2")
// res11: cats.data.Tuple2K[Option,scala.util.Try,Float] = Tuple2K(Some(2.0),Success(2.0))

If you want to combine more than 2 interpreters, the @autoProductNK attribute adds a series of product{n}K (n = 3..9) methods to the companion object. Unlike productK living in the SemigroupalK type class, currently we don't have a type class for these product{n}K operations yet.

@autoFunctor, @autoInvariant and @autoContravariant

Cats-tagless also provides three annotations that can generate cats.Functor, cats.Invariant cats.Contravariant instance for traits.

For documentation/FAQ/guides, go to typelevel.github.io/cats-tagless.

Community

Any contribution is more than welcome. Also feel free to report bugs, request features using GitHub issues or gitter.

Discussion around Cats-tagless is encouraged in the Gitter channel as well as on GitHub issue and PR pages.

We adopted the Scala Code of Conduct. People are expected to follow it when discussing Cats-tagless on the GitHub page, Gitter channel, or other venues.

Maintainers

• Georgi Krastev
• Kailuo Wang
• Luka Jacobowitz

Copyright

Copyright (C) 2019 Maintainers of Cats-tagless

License

Cats-tagless is licensed under the Apache License 2.0