Simple example of browser E2E tests using free monad

BrowserE2E.scala

package example

import cats.effect.IO
import cats.free.Free
import cats.free.Free.liftF
import cats.implicits._
import cats.~>

object BrowserE2E extends App {

  // Browser algebra / DSL
  sealed trait BrowserA[A]
  case class Navigate(url: Url) extends BrowserA[Document]
  case class Click(document: Document, point: Point) extends BrowserA[Element]
  case class Inspect(document: Document, query: Query) extends BrowserA[Option[Element]]

  // "Free" the algebra
  type Browser[A] = Free[BrowserA, A]

  // Helpers (not required)
  def navigate(url: Url): Browser[Document] =
    liftF(Navigate(url))

  def click(document: Document, point: Point): Browser[Element] =
    liftF(Click(document, point))

  def inspect(document: Document, query: Query): Browser[Option[Element]] =
    liftF(Inspect(document, query))

  // Simple test program (just a data structure)
  val test: Browser[Boolean] = for {
    doc <- navigate(Url("http://localhost:3000"))
    _ <- click(doc, Point(100, 100))
    header <- inspect(doc, Query("h1#header"))
  } yield header match {
    case Some(h1) => h1.text == "hello"
    case None => false
  }

  // Interpret each operation in the algebra into an IO
  // This is where you actually perform sync / async IO
  val interpreter = new (BrowserA ~> IO) {
    override def apply[A](fa: BrowserA[A]): IO[A] = fa match {
      case Navigate(url) => IO(println(s"Navigating to $url")).as(Document().asInstanceOf[A])
      case Click(document, point) => IO(println(s"Clicking on $document at $point")).as(Element().asInstanceOf[A])
      case Inspect(document, query) => IO(println(s"Quering $document for $query")).as(Some(Element()).asInstanceOf[A])
    }
  }

  // Run the test with the given interpreter (end of the world)
  println(s"Test Result: ${test.foldMap(interpreter).unsafeRunSync()}")

}

// Fake domain objects
case class Url(url: String)
case class Document(/* ... */)
case class Element(/* ... */) {
  def text: String = "hello"
}
case class Point(x: Int, y: Int)
case class Query(query: String)