Monad Transformers and Cats MTL

July 2019

There are various ways to combine effects encoded in monads. One of them is monad transformers which help to stack up different monads into one. However, there’re some drawbacks of monad transformers such as complexity. In this article, Cats MTL is used for reducing this complexity.


If you’re not familiar with the monad transformers, you can find a nice article about the concept here. There’s also a very comprehensive Typelevel blog article about Cats MTL.

To create an example, let’s think about a use case. Suppose there is a requirement for querying users from a repository. In addition, the application should return how many times a particular user is queried. First let’s try to find monads corresponding to required effects, and use monad transformers to combine them. For simplicity, assume that it’s not necessary to think about IO interaction at this point. Rather than, this post will focus on how to select the repository and store the query counts. The problem will be stated after finishing the initial implementation.


  • Scala: 2.13.0
  • cats-core: 2.0.0
  • cats-mtl-core: 0.6.0
  • kind-projector: 1.2.0


To begin, let’s define the User record.

type UserId = Long
final case class User(id: UserId, name: String)

Next is defining types for failure. Since users will be queried from some repository, it’s possible that some users might not be found.

final case class Failure(message: String)
type EitherFailure[A] = Failure Either A

Using EitherFailure, now we can define an interface for querying.

trait Db {
  def user(id: UserId): EitherFailure[User]

It would be good to be able to select the Db instance based on some environment. Reader monad perfectly suits for this job. Reader can be used for fetching a read-only state (such as configuration) as well as dependency injection. And here it’s utilised for the latter practice. Because Either is already in the playground, machinery is needed to combine both effects. What’s required is exactly the ReaderT.

type ReaderTEither[A, B] = ReaderT[EitherFailure, A, B]

Not surprisingly, ReaderT is just an alias for Kleisli in Cats. Moreover, Reader is defined as simply setting Id as the inner type.

type ReaderT[F[_], A, B] = Kleisli[F, A, B]
type Reader[A, B] = ReaderT[Id, A, B]

Now let’s define a Service interface which will read Db instance from some environment and query the user by id.

trait Service {
  def query(id: UserId): ReaderTEither[Db, User] = ReaderT(_.user(id))

import cats.implicits._
object Program1 extends Service {
  def result: ReaderTEither[Db, (User, User)] = for {
    user1 <- query(1L)
    user2 <- query(2L)
  } yield (u1, u2)

Above, result returns a function (a Kleisli arrow to be precise) which takes Db as parameter and returns pair of User instances wrapped in Either. For given Program1, the paired instances will have id 1 and id 2 respectively.

What happens if there’s no user with id 1? Thanks to combined effect of ReaderTEither, whenever a Failure occurs, for comprehension will short-circuit immediately. To see it’s working, let’s create a dummy Db implementation and run what’s returned by result.

def inMemoryDb = new Db {

  val users = Map[UserId, User](
    1L -> User(1L, "Rick"),
    2L -> User(2L, "Morty"),
    3L -> User(3L, "Summer")

  override def user(id: UserId): EitherFailure[User] =
    users.get(id).map(Right(_)).getOrElse(Left(Failure(s"No user found with id($id)")))

val p1 =

Note that, run gives back the composed Kleisli arrow, which needs to be evaluated by concrete inMemoryDb. Here’s the output:


Since all the users are found, we see it Right.

This was the first part of the requirements. Question is, how to add the query counts on the top this implementation? There needs to be some context, where the program logic can read and write query counts as it processes through. Well, the answer is the State monad. But to add the stateful computation effect to the already populated ReaderTEither monad, we need StateT. First, let’s define the data structure to hold the query counts.

type UserQueryCount = Map[UserId, Long]

Next is to define StateTReaderTEither, which encapsulates ReaderTEither. Here I’ve used kind-projector plugin to make types more readable. In the following type definition, we need to keep ReaderTEither as a container with a single type variable, which is achieved by type lambda (?).

type StateTReaderTEither[A, B] = StateT[ReaderTEither[A, ?], UserQueryCount, B]

Having this new type, now we can rewrite the Service class with utilising provided UserQueryCount state. Before let’s define QueryResult type which is a product of user and how many times it’s queried so far.

type QueryResult = (User, Long)

trait Service {
  def query(id: UserId): StateTReaderTEither[Db, QueryResult] = for {
    queryCounts <- StateT.get[ReaderTEither[Db, ?], UserQueryCount]
    count       =  queryCounts.getOrElse(id, 0L) + 1L
    _           <- StateT.set[ReaderTEither[Db, ?], UserQueryCount] {
                     queryCounts + (id -> count)
    reader      =  ReaderT[EitherFailure, Db, QueryResult] {
                     r => r.user(id).map(u => (u, count))
    result      <- StateT.apply[ReaderTEither[Db, ?], UserQueryCount, QueryResult] {
                     s => => (s, r))
  } yield result

Let’s go step by step. The return type is QueryResult wrapped in StateTReaderTEither. Inside the for comprehension, first 3 line gets the state (binding to queryCounts), increments the count by 1 and sets the state with updated Map. Notice the noise around the types while calling the get and set functions on StateT. Next line queries the Db, constructs QueryResult by mapping over EitherFailure[User], and wraps the value in ReaderT. Finally, StateT is constructed via apply taking f: UserQueryCount => ReaderTEither[Db, (UserQueryCount, QueryResult)].

Again, what happens if the user with given id not found? For comprehension will short-circuit while performing the query on Db. So we have all the effects in one place. Here’s Program2 using the Service created above.

object Program2 extends Service {
  def result: StateTReaderTEither[Db, QueryResult] = for {
    _  <- query(1L)
    _  <- query(1L)
    u2 <- query(2L)
  } yield u2

val p2 =

Note that the first run runs State monad with an initial state (here an empty Map). The second run, on the other hand, evaluates the function returned by Reader monad with inMemoryDb. Let’s try to run it and see what happens.

Right((Map(1 -> 2, 2 -> 1),(User(2,Morty),1)))

Because State is a function returning a product of state and the result, we see both of them in the output. The state, namely UserQueryCount map, shows that Rick (id 1) is queried 2 times and Morty (id 2) is queried 1 time. The second part of the product is another product, and since the last query was made against id 2, it gives the user Morty and its query count. Finally, as all the users are found in inMemoryDb, the entire value is Right.

To sum up, with the help of the monad transformers StateT and ReaderT, we managed to put all effects in one place and implement the requirements in a functional way.


It’s easily noticed that we need to explicitly put type annotations in the previous implementation since Scala compiler cannot infer them completely.

This can be cumbersome when the transformer stack consists of multiple layers. More monad transformers, worse type inference. If the combined monad effects are widely used in the code base, having noisy type annotations will increase the development time.

Being said, Cats MTL library provides a cleaner way to deal with multiple effects.

Cats MTL

Cats MTL substantiates the idea of encoding the effects in type classes rather than data structures. It helps to write the code against some type constructor F[_] fulfilling different type class constraints.

There are several type classes provided by the Cats MTL library to simulate most common monadic effects. You can find the entire list here. One nice thing about the type classes is that they stick to the principle of the least power. For instance, FunctorRaise is enough to be Functor to provide functionality for raising errors. In addition, to avoid implicit ambiguities, Cats MTL library favors composition for such constraints.

Let’s try to re-implement the requirements with using MTL library this time. To meet the requirements, we need 3 different effects: read-write state (StateT), read-only state (ReaderT) and error handling (Either). For the first two, we can pick MonadState and ApplicativeAsk. For the last effect, let’s select MonadError to start with. New Service class will look like this:

import cats.MonadError
import cats.mtl.{ApplicativeAsk, MonadState}

trait Service {
  def query[F[_]](id: UserId)(
    implicit S: MonadState[F, UserQueryCount],
             A: ApplicativeAsk[F, Db],
             E: MonadError[F, Failure]
    ): F[QueryResult] = for {
    queryCounts <- S.get
    count       =  queryCounts.getOrElse(id, 0L) + 1L
    _           <- S.set(queryCounts + (id -> count))
    result      <- E.rethrow(A.reader(_.user(id).map(u => (u, count))))
  } yield result

Method declaration includes implicit parameters which witnesses that selected F[_] complies with required effects. Inside the for comprehension, state is fetched with simply calling S.get. Because the type F[S] can easily inferred by the Scala compiler, there’s no need to explicitly set type annotations. This is very convenient compared to the previous implementation. Next, state is updated with the incremented query count. The last line is even more concise. With the help of rethrow function on MonadError, we could easily pass the Either value wrapped in F[_].

Note that query is not ready to be run yet. So far we only specified the required effects but didn’t provide the actual F[_] implementation meeting all the requirements.

Luckily, Cats MTL provides a way to materialize from a monad transformer stack. With using a bunch of implicits defined under cats.mtl.implicits library, we can easily create a materializedQuery function from query function by giving the entire monad stack in the type signature.

import cats.mtl.implicits._
def materializedQuery(id: UserId) =
  query[StateT[ReaderT[EitherFailure, Db, ?], UserQueryCount, ?]](id)

Now let’s run the Program3 and see what happens.

object Program3 extends Service {
  def result: StateTReaderTEither[Db, QueryResult] = for {
    _  <- materializedQuery(1L)
    _  <- materializedQuery(1L)
    u2 <- materializedQuery(2L)
  } yield u2

val p3 =
Right((Map(1 -> 2, 2 -> 1),(User(2,Morty),1)))

We’ve got exactly the same result as in the previous implementation.

One more thing to consider is that the MonadError is directly coming from cats package and it’s not a part of Cats MTL type classes. Here I’ve used for convenience, particularly for 2 reasons. Since MonadError extends Monad, F[_] could be used inside of the for comprehension, as map and flatMap functions are defined for it. Plus, rethrow is very handy when working with Either.

However, using a type class extending Monad might not fit well with the design philosophy of Cats MTL library. So let’s try to replace MonadError with FunctorRaise. However, this time we need to be sure that query signature requires a Monad constraint on F[_].

trait Service {
  def query[F[_] : Monad](id: UserId)(
    implicit S: MonadState[F, UserQueryCount],
             A: ApplicativeAsk[F, Db],
             E: FunctorRaise[F, Failure]
    ): F[(User, Long)] = for {
    queryCounts <- S.get
    count       =  queryCounts.getOrElse(id, 0L)
    _           <- S.set(queryCounts + (id -> (count + 1L)))
    reader      <- A.reader(_.user(id).map(u => (u, count)))
    result      <- reader match {
                      case Right(r) => Monad[F].pure(r)
                      case Left(e)  => E.raise[(User, Long)](e)
  } yield result

  def materializedQuery(id: UserId) =
    fetch[StateT[ReaderT[WithError, Db, ?], UserQueryCount, ?]](id)


Monad transformers are infamous for their performance characteristics. I’m not going to deep dive into the reasons, but you can find a good article here. Now let’s compare the 2 implementations created so far. To do this, jmh can be used via sbt-jmh plugin. Let’s create a benchmarking class which will call these 2 implementations 1000 times.

import cats.implicits._
import org.openjdk.jmh.annotations._

class BenchmarkTests {

  def transformers(count: Int): Unit =
    for (_ <- 0 until count)

  def mtl(count: Int): Unit =
    for (_ <- 0 until count)

  def transformersBenchmark: Unit = transformers(1000)

  def mtlBenchmark: Unit = mtl(1000)

Benchmark report will give scores as average operation times in seconds. Here’s the result after running 10 warmup and 10 measurement iterations.

jmh:run -i 10 -wi 10 -f1 -t1 ... [info] Result "mtl.BenchmarkTests.mtlBenchmark": [info] 0.004 ±(99.9%) 0.001 s/op [Average] [info] (min, avg, max) = (0.004, 0.004, 0.005), stdev = 0.001 [info] CI (99.9%): [0.004, 0.005] (assumes normal distribution) ... [info] Result "mtl.BenchmarkTests.transformersBenchmark": [info] 0.003 ±(99.9%) 0.001 s/op [Average] [info] (min, avg, max) = (0.003, 0.003, 0.003), stdev = 0.001 [info] CI (99.9%): [0.003, 0.003] (assumes normal distribution) ... [info] Do not assume the numbers tell you what you want them to tell. [info] Benchmark Mode Cnt Score Error Units [info] BenchmarkTests.mtlBenchmark avgt 10 0.004 ± 0.001 s/op [info] BenchmarkTests.transformersBenchmark avgt 10 0.003 ± 0.001 s/op

We observe similar performance characteristics. This is because we materialize F[_] over the monad transformer stack. However, this is not the only way while using the Cats MTL type classes. Another nice thing about Cats MTL is that there’s a clear separation between 2 aspects it consists of: definition of type classes and interpretation. As you can see, program logic can be implemented without providing the interpreter. So that performance can be improved with different interpreters.


Cats MTL significantly reduces the complexity introduced by monad transformers. We can summarize the benefits as:

  • Fewer type annotations.
  • Support for most common effects.
  • Implicit materialization for monad transformers.


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