Cats-tagless - Scala macro annotation error

Question

I have created a simple trait using cats-tagless lib:

@finalAlg
@autoFunctorK(true)
trait MyService[F[_]] {

  def put(element: Element): F[Element]

  def get(elementId: Id): F[Element]

  def all(): F[List[Element]]

  def delete(elementId: Id): F[Unit]
}

but when I tried to compile it, I got an error:

Error:(8, 7) macro annotation could not be expanded (the most common reason for that is that you need to enable the macro paradise plugin; another possibility is that you try to use macro annotation in the same compilation run that defines it)

I also added addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.1" cross CrossVersion.full) into plugins.sbt file and build.sbt but it did not help. Can you help me to solve it?

My build.sbt file looks like:

addCompilerPlugin(("org.scalameta" % "paradise" % "3.0.0-M11").cross(CrossVersion.full))

lazy val commonSettings = Seq(
  libraryDependencies ++= Seq(
    "org.typelevel" %% "cats-core" % CatsVersion,
    "org.typelevel" %% "cats-effect" % "1.2.0",
    "org.typelevel" %% "cats-tagless-macros" % "0.5",
    "org.typelevel" %% "cats-tagless-legacy-macros" % "0.5",
    "org.typelevel" %% "cats-mtl-core" % "0.5.0",
  )
)

Answer 1

In an empty new project with this build.sbt:

scalaVersion := "2.12.8"

libraryDependencies ++= Seq(
  "org.typelevel" %% "cats-tagless-macros" % "0.5",
  "org.typelevel" %% "cats-tagless-legacy-macros" % "0.5"
)

addCompilerPlugin(
  "org.scalameta" % "paradise" % "3.0.0-M11" cross CrossVersion.full
)

this code:

import cats.tagless._

case class Element()
case class Id()

@finalAlg
@autoFunctorK(true)
trait MyService[F[_]] {
  def put(element: Element): F[Element]
  def get(elementId: Id): F[Element]
  def all(): F[List[Element]]
  def delete(elementId: Id): F[Unit]
}

compiles just fine, as advertised here.

If I remove addCompilerPlugin("org.scalameta" % "paradise" % "3.0.0-M11" cross CrossVersion.full), I get the same error message:

macro annotation could not be expanded (the most common reason for that is that you need to enable the macro paradise plugin; another possibility is that you try to use macro annotation in the same compilation run that defines it)

Again, this is as documented, the linked page says:

The macro annotations (@finalAlg, @autoFunctorK, @autoInvariantK, etc.) still depends on scalameta, so you need to add scalameta dependencies in build.sbt.

So it seems that you need it because of @finalAlg and @autoFunctorK.

Note that I didn't modify anything in project/.

---

EDIT

If you have multiple subprojects, you have to add the compiler plugin to the subproject that actually needs it. Notice that

addCompilerPlugin(foobar)

is essentially just

libraryDependencies += compilerPlugin(foobar)

so in your case, you probably should try something like

  libraryDependencies ++= Seq(
    "org.typelevel" %% "cats-core" % "1.6.0",
    "org.typelevel" %% "cats-effect" % "1.2.0",
    "org.typelevel" %% "cats-tagless-macros" % "0.5",
    "org.typelevel" %% "cats-tagless-legacy-macros" % "0.5",
    "org.typelevel" %% "cats-mtl-core" % "0.5.0",
    compilerPlugin(("org.scalameta" % "paradise" % "3.0.0-M11")
      .cross(CrossVersion.full))
  )

and then add this to your algebra subproject.