SlickCats (Fork)

Note: This is a fork of the original SlickCats project with updated dependencies and extended compatibility matrix.

Cats instances for Slick's DBIO including:

• Monad
• MonadError
• CoflatMap
• Group
• Monoid
• Semigroup
• Comonad
• Order
• PartialOrder
• Equals

Using

This fork provides separate artifacts for different Slick versions to ensure compatibility. Add the appropriate dependency to your build definition:

// For Slick 3.3.x
libraryDependencies += "tech.engine" %% "slickcats-slick3-3" % "0.11.0-SNAPSHOT" // Scala 2.12/2.13

// For Slick 3.4.x
libraryDependencies += "tech.engine" %% "slickcats-slick3-4" % "0.11.0-SNAPSHOT" // Scala 2.12/2.13

// For Slick 3.5.x
libraryDependencies += "tech.engine" %% "slickcats-slick3-5" % "0.11.0-SNAPSHOT" // Scala 2.12/2.13/3.3.x (LTS)

// For Slick 3.6.x
libraryDependencies += "tech.engine" %% "slickcats-slick3-6" % "0.11.0-SNAPSHOT" // Scala 2.12/2.13/3.3.x (LTS)

Because of possible binary incompatibilities, here are the dependency versions used in each release:

This fork supports multiple Slick and Scala version combinations:

Artifact	Release	Slick Version	Scala Versions	Cats Version
slickcats-slick3-3	0.11.0-SNAPSHOT	3.3.3	2.12.19, 2.13.16	2.13.0
slickcats-slick3-4	0.11.0-SNAPSHOT	3.4.1	2.12.19, 2.13.16	2.13.0
slickcats-slick3-5	0.11.0-SNAPSHOT	3.5.2	2.12.19, 2.13.16, 3.3.6	2.13.0
slickcats-slick3-6	0.11.0-SNAPSHOT	3.6.1	2.12.19, 2.13.16, 3.3.6	2.13.0

Artifacts are publicly available on Maven Central starting from version 0.6.

Accessing the Instances

Some or all of the following imports may be needed:

import cats._
import slick.dbio._
import com.rms.miu.slickcats.DBIOInstances._

Additionally, be sure to have an implicit ExecutionContext in scope. The implicit conversions require it and will fail with non-obvious errors if it's missing.

implicitly[Monad[DBIO]]
// error:
// No given instance of type cats.Monad[slick.dbio.DBIO] was found for parameter e of method implicitly in object Predef.
// I found:
// 
//     com.rms.miu.slickcats.DBIOInstances.dbioInstance(
//       /* missing */summon[scala.concurrent.ExecutionContext])
// 
// But no implicit values were found that match type scala.concurrent.ExecutionContext.
// def monad[F[_] : Monad, A](fa: F[A]): F[A] = fa
//

import scala.concurrent.ExecutionContext.Implicits.global

instances will be available for:

implicitly[Monad[DBIO]]
implicitly[MonadError[DBIO, Throwable]]
implicitly[CoflatMap[DBIO]]
implicitly[Functor[DBIO]]
implicitly[Applicative[DBIO]]

If a Monoid exists for A, here taken as Int, then the following is also available

implicitly[Group[DBIO[Int]]]
implicitly[Semigroup[DBIO[Int]]]
implicitly[Monoid[DBIO[Int]]]

Known Issues

Instances are supplied for DBIO[A] only. Despite being the same thing, type aliases will not match for implicit conversion. This means that the following

def monad[F[_] : Monad, A](fa: F[A]): F[A] = fa

val fail1: DBIOAction[String, NoStream, Effect.All] = DBIO.successful("hello")
// fail1: DBIOAction[String, NoStream, All] = SuccessAction(value = "hello")
val fail2 = DBIO.successful("hello")
// fail2: DBIOAction[String, NoStream, Effect] = SuccessAction(value = "hello")
val success: DBIO[String] = DBIO.successful("hello")
// success: DBIOAction[String, NoStream, All] = SuccessAction(value = "hello")

will not compile

monad(fail1)
monad(fail2)
// error:
// Found:    (repl.MdocSession.MdocApp.fail1 :
//   slick.dbio.DBIOAction[String, slick.dbio.NoStream, slick.dbio.Effect.All])
// Required: ([_] =>> Any)[Any]
// Note that implicit conversions were not tried because the result of an implicit conversion
// must be more specific than ([_] =>> Any)[Any]
// monad(fail1)
//       ^^^^^
// error:
// Found:    (repl.MdocSession.MdocApp.fail2 :
//   slick.dbio.DBIOAction[String, slick.dbio.NoStream, slick.dbio.Effect])
// Required: ([_] =>> Any)[Any]
// Note that implicit conversions were not tried because the result of an implicit conversion
// must be more specific than ([_] =>> Any)[Any]
// monad(fail2)
//       ^^^^^

but

monad(success)
// res10: DBIOAction[String, NoStream, All] = SuccessAction(value = "hello")

will compile fine.

Extras

This README is compiled using mdoc to ensure that only working examples are given.