Symbol 'type cats.MonadFilter' is missing fromthe classpath

Question

I am reading this tutorial on tagless final.

Based on this I have defined my dependencies as

object Dependencies {
  lazy val scalaTest = "org.scalatest" %% "scalatest" % "3.0.5"
  lazy val cats = "org.typelevel" %% "cats-core" % "1.2.0"
  lazy val monix = "io.monix" %% "monix" % "2.3.3"
  lazy val monixCats = "io.monix" %% "monix-cats" % "2.3.3"
}

The following is my code

// future
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent._
import scala.concurrent.duration._

// cats
import cats.Monad
import cats.implicits._

// monix
import monix.eval.Task
import monix.cats._
import monix.cats.reverse._


trait ProductRepository[M[_]] {
  def findProduct(productId: ProductId) : M[Option[Product]]
  def saveProduct(product: Product) : M[Unit]
  def incrementProductSales(productId: ProductId, quantity: Long) : M[Unit]
}

class ProductRepositoryWithFuture extends ProductRepository[Future] {
  def findProduct(productId: ProductId) : Future[Option[Product]] = {
    Future.successful(Some(Product(productId, "foo")))
  }
  def saveProduct(product: Product) : Future[Unit] = {
    Future.successful()
  }
  def incrementProductSales(productId: ProductId, quanity: Long) : Future[Unit] = {
    Future.successful()
  }
}

class ProductRepositoryWithTask extends ProductRepository[Task] {
  def findProduct(productId: ProductId) : Task[Option[Product]] = {
    Task.now(Some(Product(productId, "foo")))
  }
  def saveProduct(product: Product) : Task[Unit] = {
    Task.unit
  }
  def incrementProductSales(productId: ProductId, quantity: Long) : Task[Unit] = {
    Task.unit
  }
}

But I get bunch of errors. It seems that the version of cats which I am using is not compatible with the one Monix uses.

I also tried to remove my cats dependency and just imported monix so that monix pulls in its own version of cats. but even that doesn't compile.

error] /Users/foobar/code/tagless/src/main/scala/example/Hello.scala:54:24: Symbol 'type cats.MonadFilter' is missing fromthe classpath.
[error] This symbol is required by 'method monix.cats.MonixToCatsCore7.monixToCatsMonadFilter'.
[error] Make sure that type MonadFilter is in your classpath and check for conflicting dependencies with `-Ylog-classpath`.
[error] A full rebuild may help if 'MonixToCatsCore7.class' was compiled against an incompatible version of cats.
[error]       repo.findProduct(id).flatMap{
[error]                        ^
[error] /Users/foobar/code/tagless/src/main/scala/example/Hello.scala:54:23: diverging implicit expansion for type monix.types.Comonad[M]
[error] starting with method catsToMonixComonad in trait CatsCoreToMonix5
[error]       repo.findProduct(id).flatMap{
[error]                       ^
[error] /Users/foobar/code/tagless/src/main/scala/example/Hello.scala:54:28: value flatMap is not a member of type parameter M[Option[example.Application.Product]]
[error]       repo.findProduct(id).flatMap{
[error]                            ^
[error] /Users/foobar/code/tagless/src/main/scala/example/Hello.scala:56:30: value copy is not a member of Any
[error]           val newProduct = p.copy(name = name)
[error]                              ^
[error] /Users/foobar/code/tagless/src/main/scala/example/Hello.scala:56:40: reassignment to val
[error]           val newProduct = p.copy(name = name)
[error]                                        ^
[error] /Users/foobar/code/tagless/src/main/scala/example/Hello.scala:57:27: diverging implicit expansion for type monix.types.MonadError[M,E]
[error] starting with method catsToMonixMonadError in trait CatsCoreToMonix3
[error]           repo.saveProduct(newProduct).map(_ => Some(p))
[error]                           ^
[error] /Users/foobar/code/tagless/src/main/scala/example/Hello.scala:57:40: value map is not a member of type parameter M[Unit]
[error]           repo.saveProduct(newProduct).map(_ => Some(p))
[error]                                        ^
[error] /Users/foobar/code/tagless/src/main/scala/example/Hello.scala:59:16: diverging implicit expansion for type cats.Comonad[M]
[error] starting with method monixToCatsComonad in trait MonixToCatsCore5
[error]           Monad[M].pure(None)
[error]                ^
[error] 8 errors found

Answer 1

The errors are caused by incompatibilities between your dependencies. For example monix 2.3.3 depends on cats 0.9.0 while you're trying to use 1.2.0 which is binary incompatible.

You should try either upgrading monix to 3.x or downgrading cats to 0.9.0.

P.S. The transition from cats 0.9.0 to 1.x has a lot of breaking changes and you have to make sure that all libraries you're using are compiled against the same (or at least binary compatible) version of cats.