JarCastingSlickCats
Cats instances for Slick's DBIO including:
- Monad
- MonadError
- CoflatMap
- Group
- Monoid
- Semigroup
- Comonad
- Order
- PartialOrder
- Equals
Using
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:
| slick-cats version | slick version | cats version |
|---|---|---|
| 0.10.4 | 3.3.3 | 2.3.1 |
| 0.10.3 | 3.3.2 | 2.2.0 |
| 0.10.2 | 3.3.2 | 2.1.0 |
| 0.10.1 | 3.3.2 | 2.0.0 |
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: Could not find an instance of Monad for slick.dbio.DBIO
// 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]]]
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, Effect.All] = SuccessAction("hello")
val fail2 = DBIO.successful("hello")
// fail2: DBIOAction[String, NoStream, Effect] = SuccessAction("hello")
val success: DBIO[String] = DBIO.successful("hello")
// success: DBIO[String] = SuccessAction("hello")
will not compile
monad(fail1)
monad(fail2)
// error: no type parameters for method monad: (fa: F[A])(implicit evidence$1: cats.Monad[F])F[A] exist so that it can be applied to arguments (slick.dbio.DBIOAction[String,slick.dbio.NoStream,slick.dbio.Effect.All])
// --- because ---
// argument expression's type is not compatible with formal parameter type;
// found : slick.dbio.DBIOAction[String,slick.dbio.NoStream,slick.dbio.Effect.All]
// required: ?F[?A]
// monad(fail1)
// ^^^^^
// error: type mismatch;
// found : slick.dbio.DBIOAction[String,slick.dbio.NoStream,slick.dbio.Effect.All]
// required: F[A]
// monad(fail1)
// ^^^^^
// error: no type parameters for method monad: (fa: F[A])(implicit evidence$1: cats.Monad[F])F[A] exist so that it can be applied to arguments (slick.dbio.DBIOAction[String,slick.dbio.NoStream,slick.dbio.Effect])
// --- because ---
// argument expression's type is not compatible with formal parameter type;
// found : slick.dbio.DBIOAction[String,slick.dbio.NoStream,slick.dbio.Effect]
// required: ?F[?A]
// monad(fail2)
// ^^^^^
// error: type mismatch;
// found : slick.dbio.DBIOAction[String,slick.dbio.NoStream,slick.dbio.Effect]
// required: F[A]
// monad(fail2)
// ^^^^^
but
monad(success)
// res10: DBIO[String] = SuccessAction("hello")
will compile fine.
Extras
This README is compiled using mdoc to ensure that only working examples are given.
