Example of free tagless combined handlers for module

CombiningTaglessHandlers.scala

import cats.data.IdT
import cats.{~>, Id, Monad}
import freestyle.tagless.logging.LoggingM
import freestyle.tagless._
import sourcecode.{File, Line}

@tagless trait Summer {
  def sum(a: Int, b: Int): FS[Int]
}

@module trait Application {
  implicit val M: Monad[FS]
  import cats.syntax.flatMap._
  import cats.syntax.functor._

  val logging: LoggingM
  val summer: Summer

  def program: FS[Int] =
    for {
      _   <- logging.debug("test1")
      _   <- logging.debug("test2")
      _   <- logging.debug("test3")
      sum <- summer.sum(5, 3)
    } yield sum
}

/* handlers */
trait SummerHandler {
  implicit val summerHandler = new Summer.Handler[Id] {
    def sum(a: Int, b: Int): FS[Int] = a + b
  }
}

trait LoggingHandler {
  implicit val logHandlerOption: LoggingM.Handler[Option] = new LoggingM.Handler[Option] {
    def debug(msg: String, sourceAndLineInfo: Boolean)(implicit line: Line, file: File): Option[Unit]                            = Some(())
    def debugWithCause(msg: String, cause: Throwable, sourceAndLineInfo: Boolean)(implicit line: Line, file: File): Option[Unit] = Some(())
    def error(msg: String, sourceAndLineInfo: Boolean)(implicit line: Line, file: File): Option[Unit]                            = Some(())
    def errorWithCause(msg: String, cause: Throwable, sourceAndLineInfo: Boolean)(implicit line: Line, file: File): Option[Unit] = Some(())
    def info(msg: String, sourceAndLineInfo: Boolean)(implicit line: Line, file: File): Option[Unit]                             = Some(())
    def infoWithCause(msg: String, cause: Throwable, sourceAndLineInfo: Boolean)(implicit line: Line, file: File): Option[Unit]  = Some(())
    def warn(msg: String, sourceAndLineInfo: Boolean)(implicit line: Line, file: File): Option[Unit]                             = Some(())
    def warnWithCause(msg: String, cause: Throwable, sourceAndLineInfo: Boolean)(implicit line: Line, file: File): Option[Unit]  = Some(())
  }
}

trait CombinedHandler extends SummerHandler with LoggingHandler {
  // given a natural transformation Option ~> Id we can create a new handler for Id from Option handler
  implicit val logHandlerId: LoggingM[Id] =
    logHandlerOption.mapK(new (Option ~> Id) {
      override def apply[A](optionA: Option[A]): Id[A] = IdT.pure(optionA.get).value
    })
}

object CombiningTaglessHandlers extends App with CombinedHandler {
  val result = Application[Id].program

  println(s"result is $result")
  // result is 8
}