Generate apply methods creating a class

Question

Scala 2.13

I have tons of similar traits of the form

trait SomeTrait[F[_]]{
    def someOp(): F[Unit]
    //...
}

and their implementations

class SomeTraitImpl[F[_]: Sync] extends SomeTrait[F]{
   //...
}

object SomeTrait{
    def apply[F[_]: Sync](): SomeTrait[F] = new SomeTraitImpl[F]()
}

The problem is such companion with the single apply method looks pretty ugly and it is a boilerplate. Is there a way to automate the object generation? Can simulacrum or anything else (a hand written macro annotation?) do that?

Answer 1

You can use macro annotation

import scala.annotation.{StaticAnnotation, compileTimeOnly}
import scala.language.experimental.macros
import scala.reflect.macros.blackbox

@compileTimeOnly("enable macro paradise")
class implApply extends StaticAnnotation {
  def macroTransform(annottees: Any*): Any = macro ImplApplyMacro.macroTransformImpl
}

object ImplApplyMacro {
  def macroTransformImpl(c: blackbox.Context)(annottees: c.Tree*): c.Tree = {
    import c.universe._

    def applyMethod(tparams: Seq[Tree], tpname: TypeName): Tree = {
      def tparamNames = tparams.map {
        case q"$_ type $tpname[..$_] = $_" => tq"$tpname"
      }
      q"""def apply[..$tparams]()(implicit sync: Sync[${tparamNames.head}]): $tpname[..$tparamNames] =
            new ${TypeName(tpname + "Impl")}[..$tparamNames]()"""
    }

    annottees match {
      case (trt@q"$_ trait $tpname[..$tparams] extends { ..$_ } with ..$_ { $_ => ..$_ }") ::
        q"$mods object $tname extends { ..$earlydefns } with ..$parents { $self => ..$body }" :: Nil =>
        q"""
          $trt
          $mods object $tname extends { ..$earlydefns } with ..$parents { $self =>
            ${applyMethod(tparams, tpname)}
            ..$body
          }
        """

      case (trt@q"$_ trait $tpname[..$tparams] extends { ..$_ } with ..$_ { $_ => ..$_ }") :: Nil =>
        q"""
          $trt
          object ${tpname.toTermName} {
            ${applyMethod(tparams, tpname)}
          }
        """
    }
  }
}

Usage:

@implApply
trait SomeTrait[F[_]]{
  def someOp(): F[Unit]
}

class SomeTraitImpl[F[_]: Sync] extends SomeTrait[F]{
  override def someOp(): F[Unit] = ???
}

//Warning:scalac: {
//  object SomeTrait extends scala.AnyRef {
//    def <init>() = {
//      super.<init>();
//      ()
//    };
//    def apply[F[_]]()(implicit sync: Sync[F]): SomeTrait[F] = new SomeTraitImpl[F]()
//  };
//  ()
//}