Cats instances for Slick's DBIO including:

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

To add slick-cats dependency to a project, add the following to your build definition:

libraryDependencies += "com.rms.miu" %% "slick-cats" % version

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

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

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]]

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]]]

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")
val fail2 = DBIO.successful("hello")
val success: DBIO[String] = DBIO.successful("hello")

will not compile

monad(fail1)
monad(fail2)

but

monad(success)

will compile fine.

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