sample code for Cats MTL MonadState

monadState.sc

import cats.Monad
import cats.data.StateT
import cats.effect.{IO, Sync}
import cats.mtl.MonadState
import cats.syntax.show._
import cats.instances.int._
import cats.syntax.flatMap._
import cats.syntax.functor._
import cats.syntax.applicative._
import cats.syntax.traverse._
import cats.instances.list._
import cats.mtl.instances.state._

type Cache = Map[Int, Int]

def triple[F[_]: Sync](n: Int): F[Int] =
  Sync[F].delay { println(show"triple($n)") } as (n * 3)

def tripleWithCache[F[_]: Monad: Sync](n: Int)(implicit F: MonadState[F, Cache]): F[Int] =
  for {
    memo  <- F.get
    value <- memo.get(n) match {
      case Some(v) => v.pure[F]
      case None    => triple[F](n) >>= { v => F.modify(_.updated(n, v)) as v }
    }
  } yield value

List(1, 3, 1, 2, 3)
  .traverse(tripleWithCache[StateT[IO, Cache, ?]])
  .run(Map(3 -> 9))
  .unsafeRunSync()
// triple(1)
// triple(2)
// (Map(3 -> 9, 1 -> 3, 2 -> 6), List(3, 9, 3, 6, 9))