StateT and Gen

statefulGen.scala

// resolvers += "Typesafe Repository" at "http://repo.typesafe.com/typesafe/releases/"

// libraryDependencies += "org.scalatest" %% "scalatest" % "3.0.0"
// libraryDependencies += "org.typelevel" %% "cats" % "0.9.0"
// libraryDependencies += "io.github.amrhassan" %% "scalacheck-cats" % "0.3.2" % Test


package ex

import cats._
import cats.data._
import cats.implicits._
import org.scalacheck.Gen
import org.scalacheck.support.cats._
import org.scalatest._
import org.scalatest.prop.GeneratorDrivenPropertyChecks

class ExampleSpec extends FlatSpec with Matchers with GeneratorDrivenPropertyChecks with SG {

  case class InnerState(total: Int, floor: Int)

  def step(i: InnerState): Gen[List[Int]] = {
    if (i.floor == Int.MaxValue || i.total == 0) {
      Gen.const(List.empty)
    } else {
      for {
        head: Int <- Gen.choose(i.floor, Int.MaxValue)
        tail <- step(InnerState(i.total - 1, head))
      } yield head +: tail
    }
  }

  def stepT: StateT[Gen, InnerState, List[Int]] = for {
    i <- StateT.get[Gen, InnerState]
    r <- if (i.floor == Int.MaxValue || i.total == 0) {
      StateT.lift[Gen, InnerState, List[Int]](Gen.const(List.empty[Int]))
    } else {
      for {
        nxt <- StateT.lift(Gen.choose(i.floor, Int.MaxValue))
        _ <- StateT.set[Gen, InnerState](InnerState(i.total - 1, nxt))
        tail <- stepT
      } yield nxt +: tail
    }
  } yield r


  def gen(amt: Int) = step(InnerState(amt, 0))

  def genT(amt: Int) = stepT.runA(InnerState(amt, 0))


  "plain" should "generate sorted array" in {
    forAll(gen(10)) { l =>
      println(l)
      l.sorted shouldBe l
    }
  }

  "transformed" should "generate sorted array" in {
    forAll(genT(10)) { l =>
      println(l)
      l.sorted shouldBe l
    }

    forAll(stepT.runA(InnerState(10, Int.MaxValue - 5))) { l =>
      println(l)
      l.sorted shouldBe l
    }
  }
}