StateT with Either for error handling

StateTEither.scala

import cats._
import cats.implicits._
import cats.data.StateT

object Radio {
    case class Radio(volume: Int)

  type RadioError = String
  type RadioEither[A] = Either[RadioError, A]
  type RadioStateEitherT[A] = StateT[RadioEither, Radio, A]

  def changeVolume(v: Int) = StateT[RadioEither, Radio, Int] {
    radioState => 
      val newVolume = radioState.volume + v
      if(newVolume >= 0)
        Either.right((Radio(newVolume), newVolume))
      else
        Either.left("User set negative volume")
  }

  def main(args: Array[String]): Unit = {
    
    val operateRadio = for (
      volume <- changeVolume(10)
    ) yield volume

    println(operateRadio.run(Radio(10)))

    val badOperateRadio = for (
      volume <- changeVolume(-20)
    ) yield volume

    println(badOperateRadio.run(Radio(10)))
  }
}