Scala example source code file (StateTTest.scala)

This example Scala source code file (StateTTest.scala) is included in the alvinalexander.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Scala by Example" ^TM.

Learn more about this Scala project at its project page.

Java - Scala tags/keywords

bindrec, boolean, functor, int, list, monad, monadplus, monadstate, statet, statetlist

The StateTTest.scala Scala example source code

package scalaz

import scalaz.scalacheck.ScalazProperties._
import scalaz.scalacheck.ScalazArbitrary
import std.AllInstances._

object StateTTest extends SpecLite {

  type StateTList[S, A] = StateT[List, S, A]
  type StateTListInt[A] = StateTList[Int, A]

  implicit def stateTListEqual = Equal[List[(Int, Int)]].contramap((_: StateTListInt[Int]).runZero[Int])
  implicit def stateTListArb = ScalazArbitrary.stateTArb[List, Int, Int]
  implicit def stateTListArb2 = ScalazArbitrary.stateTArb[List, Int, Int => Int]

  checkAll(equal.laws[StateTListInt[Int]])
  checkAll(bindRec.laws[StateTListInt])
  checkAll(monad.laws[StateTListInt])

  object instances {
    def functor[S, F[_] : Functor] = Functor[StateT[F, S, ?]]
    def plus[F[_]: Monad: Plus, S1, S2] = Plus[IndexedStateT[F, S1, S2, ?]]
    def bindRec[S, F[_] : Monad : BindRec] = BindRec[StateT[F, S, ?]]
    def monadState[S, F[_] : Monad] = MonadState[StateT[F, S, ?], S]
    def monadPlus[S, F[_]: MonadPlus] = MonadPlus[StateT[F, S, ?]]

    // F = Id
    def functor[S] = Functor[State[S, ?]]
    def monadState[S] = MonadState[State[S, ?], S]

    // checking absence of ambiguity
    def functor[S, F[_] : Monad] = Functor[StateT[F, S, ?]]
    def plus[F[_]: MonadPlus, S] = Plus[StateT[F, S, ?]]
  }

  "monadState.state" in {
    instances.monadState[Boolean].state(42).run(true) must_===((true, 42))
  }

  "monadState.constantState" in {
    instances.monadState[Boolean].constantState(42, false).run(true) must_===((false, 42))
  }

  "monadState.get" in {
    instances.monadState[Boolean].get.run(true) must_===((true, true))
  }

  "monadState.gets" in {
    instances.monadState[Int].gets { _ + 1 }.run(10) must_===((10, 11))
  }

  "monadState.put" in {
    instances.monadState[Int].put(20).run(10) must_===((20, ()))
  }

  "monadState.modify" in {
    instances.monadState[Int].modify { _ + 1 }.run(10) must_===((11, ()))
  }

  "monadPlus.empty (List)" in {
    instances.monadPlus[Boolean, List].empty[Int].run(false) must_===(Nil)
  }

  "monadPlus.plus (List)" in {
    val a = StateT[List, Int, Boolean](s => List((s, false)))
    val b = StateT[List, Int, Boolean](s => List((s, true)))
    instances.monadPlus[Int, List].plus(a, b).run(0) must_===(List((0, false), (0, true)))
  }

  "StateT can be trampolined without stack overflow" in {
    import scalaz.Free._
    val result = (0 to 4000).toList.map(i => StateT[Trampoline, Int, Int]((ii:Int) => Trampoline.done((i,i))))
      .foldLeft(StateT((s:Int) => Trampoline.done((s,s))))( (a,b) => a.flatMap(_ => b))
    4000 must_=== result(0).run._1
  }
}