Mocking using Cats

CatsMocking.scala

package nm

import cats._
import cats.implicits._

import cats.data.WriterT

object Mocking {


  // a bit contrived, use your imagination :-)
  def requestThatCallsSomeServices[M[_]: Monad](f: String => M[Int], g: Int => M[Unit])(s: String): M[Unit] = {
    for {
      i <- f(s)
      _ <- g(i - 1)
      j <- f(s)
      _ <- g(j - 2)
    } yield ()
  }

  def mock = {

    type MockFramework[A] = WriterT[Id, List[String], A]

    val fMock: String => MockFramework[Int] = s => WriterT.put(s.length)(List(s"Returned ${s.length}"))
    val gMock: Int => MockFramework[Unit] = i => WriterT.tell(List(s"Function g received $i"))


    val inputString = "Test"
    val inputStringLength = inputString.length


    val expectation = List(
      s"Returned $inputStringLength",
      s"Function g received ${inputStringLength - 1}",
      s"Returned $inputStringLength",
      s"Function g received ${inputStringLength - 2}"
    )

    val runRequest: MockFramework[Unit] = requestThatCallsSomeServices(fMock, gMock)(inputString)

    val (writtenValues, _) = runRequest.run // the second param is what is returned from the function, ie Unit here

    println(writtenValues == expectation)

    /*
     * > console
     * [info] Starting scala interpreter...
     * [info]
     * Welcome to Scala version 2.11.7 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_66).
     * Type in expressions to have them evaluated.
     * Type :help for more information.
     * 
     * scala> import nm.Mocking._
     * import nm.Mocking._
     * 
     * scala> mock
     * true
     */
  }

  /*

   And then in your application code:

  def realLife = {

    val s: String = ???
    val f: String => Task[Int] = ???
    val g: Int => Task[Unit] = ???

    val result: Task[Unit] = requestThatCallsSomeServices(f, g)(s)
    // ... and so on
  }
   */
}