HasLens.scala

package data

import monocle.Lens

trait HasLens[A, B] {
  def lens: Lens[A, B]
}

object HasLens extends HasLensSyntax0

private[data] final case class HasLensOps[A, B](l: Lens[A, B]) extends AnyVal {
  def toHasLens: HasLens[A, B] = new HasLens[A, B] {
    val lens: Lens[A, B] = l
  }
}

private[data] trait HasLensSyntax0 {
  implicit def hasLensSyntax[A, B](lens: Lens[A, B]): HasLensOps[A, B] = HasLensOps(lens)
}

MonadStateLens.scala

package data

import cats.implicits._
import cats.Monad
import cats.mtl.MonadState

trait MonadStateLens[F[_], S, A] {
  def get: F[A]

  def set(a: A): F[Unit]

  def modify(f: A => A): F[Unit]

  def inspect[C](f: A => C): F[C]
}

object MonadStateLens {
  def apply[F[_], S, A](implicit FMS: MonadState[F, S], L: HasLens[S, A]): MonadStateLens[F, S, A] =
    new MonadStateLens[F, S, A] {
      implicit val m: Monad[F] = FMS.monad

      def get: F[A] =
        FMS.get.map(L.lens.get)

      def set(a: A): F[Unit] =
        FMS.get.flatMap(s => FMS.set(L.lens.set(a)(s)))

      def modify(f: A => A): F[Unit] =
        FMS.get.flatMap(s => FMS.set(L.lens.modify(f)(s)))

      def inspect[C](f: A => C): F[C] =
        FMS.get.map(s => f(L.lens.get(s)))
    }
}