StateTExtras.scala

object StateTExtras {

  // this is your brain, on drugs

  implicit class StateTTupler[F[_]: Monad, S, A](state: StateT[F, S, A]) {
    def tupled2_1[R2]: StateT[F, (S, R2), A] =
      state.transformS(_._1, (r, s) ⇒ r.copy(_1 = s))

    def tupled2_2[R1]: StateT[F, (R1, S), A] =
      state.transformS(_._2, (r, s) ⇒ r.copy(_2 = s))

    def tupled3_1[R2, R3]: StateT[F, (S, R2, R3), A] =
      state.transformS(_._1, (r, s) ⇒ r.copy(_1 = s))

    def tupled3_2[R1, R3]: StateT[F, (R1, S, R3), A] =
      state.transformS(_._2, (r, s) ⇒ r.copy(_2 = s))

    def tupled3_3[R1, R2]: StateT[F, (R1, R2, S), A] =
      state.transformS(_._3, (r, s) ⇒ r.copy(_3 = s))

    def tupled4_1[R2, R3, R4]: StateT[F, (S, R2, R3, R4), A] =
      state.transformS(_._1, (r, s) ⇒ r.copy(_1 = s))

    def tupled4_2[R1, R3, R4]: StateT[F, (R1, S, R3, R4), A] =
      state.transformS(_._2, (r, s) ⇒ r.copy(_2 = s))

    def tupled4_3[R1, R2, R4]: StateT[F, (R1, R2, S, R4), A] =
      state.transformS(_._3, (r, s) ⇒ r.copy(_3 = s))

    def tupled4_4[R1, R2, R3]: StateT[F, (R1, R2, R3, S), A] =
      state.transformS(_._4, (r, s) ⇒ r.copy(_4 = s))
  }

  def tupled2_1[S1, S2]: State[S1, ?] ~> State[(S1, S2), ?] =
    λ[State[S1, ?] ~> State[(S1, S2), ?]](_.tupled2_1)

  def tupled2_2[S1, S2]: State[S2, ?] ~> State[(S1, S2), ?] =
    λ[State[S2, ?] ~> State[(S1, S2), ?]](_.tupled2_2)

  def tupled3_1[S1, S2, S3]: State[S1, ?] ~> State[(S1, S2, S3), ?] =
    λ[State[S1, ?] ~> State[(S1, S2, S3), ?]](_.tupled3_1)

  def tupled3_2[S1, S2, S3]: State[S2, ?] ~> State[(S1, S2, S3), ?] =
    λ[State[S2, ?] ~> State[(S1, S2, S3), ?]](_.tupled3_2)

  def tupled3_3[S1, S2, S3]: State[S3, ?] ~> State[(S1, S2, S3), ?] =
    λ[State[S3, ?] ~> State[(S1, S2, S3), ?]](_.tupled3_3)

  def tupled4_1[S1, S2, S3, S4]: State[S1, ?] ~> State[(S1, S2, S3, S4), ?] =
    λ[State[S1, ?] ~> State[(S1, S2, S3, S4), ?]](_.tupled4_1)

  def tupled4_2[S1, S2, S3, S4]: State[S2, ?] ~> State[(S1, S2, S3, S4), ?] =
    λ[State[S2, ?] ~> State[(S1, S2, S3, S4), ?]](_.tupled4_2)

  def tupled4_3[S1, S2, S3, S4]: State[S3, ?] ~> State[(S1, S2, S3, S4), ?] =
    λ[State[S3, ?] ~> State[(S1, S2, S3, S4), ?]](_.tupled4_3)

  def tupled4_4[S1, S2, S3, S4]: State[S4, ?] ~> State[(S1, S2, S3, S4), ?] =
    λ[State[S4, ?] ~> State[(S1, S2, S3, S4), ?]](_.tupled4_4)
}