StateTProjections.scala

implicit class StateTProjections[F[_]: Monad, S, A](state: StateT[F, S, A]) {
  def project2_1[R2]: StateT[F, (S, R2), A] =
    state.transformS(_._1, (r, s) ⇒ r.copy(_1 = s))

  def project2_2[R1]: StateT[F, (R1, S), A] =
    state.transformS(_._2, (r, s) ⇒ r.copy(_2 = s))

  def project3_1[R2, R3]: StateT[F, (S, R2, R3), A] =
    state.transformS(_._1, (r, s) ⇒ r.copy(_1 = s))

  def project3_2[R1, R3]: StateT[F, (R1, S, R3), A] =
    state.transformS(_._2, (r, s) ⇒ r.copy(_2 = s))

  def project3_3[R1, R2]: StateT[F, (R1, R2, S), A] =
    state.transformS(_._3, (r, s) ⇒ r.copy(_3 = s))
  // ...
}