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Variance of Monad Transformers

A question that repeatedly pops up about Cats is why monad transformer types like OptionT and EitherT aren’t covariant like their Option and Either counterparts. This blog post aims to answer that question.

Covariance

What does it mean to say that Option is covariant? It means that an Option[B] is allowed to be treated as an Option[A] if B is a subtype of A. For example:

abstract class Err(val msg: String)
final case class NotFound(id: Long) extends Err(s"Not found: $id")
val optionNotFound: Option[NotFound] = Some(NotFound(42L))
// optionNotFound: Option[NotFound] = Some(NotFound(42))

optionNotFound: Option[Err]
// res0: Option[Err] = Some(NotFound(42))

Great. If you want to treat your Option[NotFound] as an Option[Err], you are free to.

This is made possible because the Option type is declared as sealed abstract class Option[+A], where the + in front of the A means that it is covariant in the A type parameter.

What happens if we try to do the same with OptionT?

import cats.Eval
import cats.data.OptionT
val optionTNotFound: OptionT[Eval, NotFound] = OptionT(Eval.now(optionNotFound))
// optionTNotFound: cats.data.OptionT[cats.Eval,NotFound] = OptionT(Now(Some(NotFound(42))))
optionTNotFound: OptionT[Eval, Err]
// <console>:17: error: type mismatch;
//  found   : cats.data.OptionT[cats.Eval,NotFound]
//  required: cats.data.OptionT[cats.Eval,Err]
// Note: NotFound <: Err, but class OptionT is invariant in type A.
// You may wish to define A as +A instead. (SLS 4.5)
//        optionTNotFound: OptionT[Eval, Err]
//        ^

The compiler complains that an OptionT[Eval, NotFound] is not an OptionT[Eval, Err], but it also suggests that we may be able to fix this by using +A like is done with Option.

OptionT in Cats is defined as:

final case class OptionT[F[_], A](value: F[Option[A]])

Let’s try to make the suggested change with an experimental MyOptionT structure:

final case class MyOptionT[F[_], +A](value: F[Option[A]])
// <console>:14: error: covariant type A occurs in invariant position in type => F[Option[A]] of value value
//        final case class MyOptionT[F[_], +A](value: F[Option[A]])
//                                             ^

Now it’s complaining that A is a covariant type but shows up in an invariant position. We’ll discuss more about what this means later in the post, but for now let’s just try declaring F as covariant:

final case class CovariantOptionT[F[+_], +A](value: F[Option[A]])
val covOptionTNotFound: CovariantOptionT[Eval, NotFound] = CovariantOptionT(Eval.now(optionNotFound))
// covOptionTNotFound: CovariantOptionT[cats.Eval,NotFound] = CovariantOptionT(Now(Some(NotFound(42))))

covOptionTNotFound: CovariantOptionT[Eval, Err]
// res2: CovariantOptionT[cats.Eval,Err] = CovariantOptionT(Now(Some(NotFound(42))))

Woohoo! Problem solved, right?

Well, not exactly. This works great if F is in fact covariant, but what if it’s not? For example, the JSON library circe has a Decoder type that could be covariant but isn’t (at least as of circe 0.10.0). With the invariant OptionT in Cats we can do something like this:

import io.circe.Decoder

def defaultValueDecoder[A](defaultValue: A, optionDecoder: Decoder[Option[A]]): Decoder[A] =
  OptionT(optionDecoder).getOrElse(defaultValue)

However, we can’t do the same with our CovariantOptionT, because we can’t even create an OptionT where the F type isn’t covariant:

def wrap[A](optionDecoder: Decoder[Option[A]]): CovariantOptionT[Decoder, A] =
  CovariantOptionT[Decoder, A](optionDecoder)
// <console>:18: error: kinds of the type arguments (io.circe.Decoder,A) do not conform to the expected kinds of the type parameters (type F,type A).
// io.circe.Decoder's type parameters do not match type F's expected parameters:
// type A is invariant, but type _ is declared covariant
//          CovariantOptionT[Decoder, A](optionDecoder)
//                          ^

In this particular case, Decoder could be declared as covariant, but it’s not. It would be unfortunate to lose the ability to use a monad transformer because a 3rd party library chose not to make a type covariant. And perhaps more importantly, sometimes you might want to use an OptionT with an F type that fundamentally isn’t covariant in nature, such as Monoid (which is invariant) or Order (which is contravariant in nature and is declared as invariant in its type definition). Later in this post there will be some examples of using OptionT with a contravariant functor type (Eq) to gain acess to a handy contramap operation.

Workaround

It’s completely reasonable to want to be able to take your OptionT[Eval, NotFound] and treat it as an OptionT[Eval, Err], and in some cases it may only be typed as OptionT[Eval, NotFound] because of type inference picking a more specific type than you intended. Luckily Cats has some handy methods to make this easy:

import cats.implicits._
optionTNotFound.widen[Err]
// res4: cats.data.OptionT[cats.Eval,Err] = OptionT(Now(Some(NotFound(42))))

The .widen method is available for any type that has a Functor. If you are working with a type that represents a Bifunctor, such as EitherT, then you can use widen for the type on the right and leftWiden for the type on the left:

import cats.data.EitherT
val eitherTNotFound: EitherT[Eval, NotFound, Some[Int]] = EitherT.leftT[Eval, Some[Int]](NotFound(42L))
// eitherTNotFound: cats.data.EitherT[cats.Eval,NotFound,Some[Int]] = EitherT(Now(Left(NotFound(42))))

eitherTNotFound.widen[Option[Int]]
// res5: cats.data.EitherT[cats.Eval,NotFound,Option[Int]] = EitherT(Now(Left(NotFound(42))))

eitherTNotFound.leftWiden[Err]
// res6: cats.data.EitherT[cats.Eval,Err,Some[Int]] = EitherT(Now(Left(NotFound(42))))

Similarly there is a narrow method for contravariant functors:

import cats.Eq
val optionTEqErr: OptionT[Eq, Err] = OptionT(Eq[Option[String]]).contramap((err: Err) => err.msg)
// optionTEqErr: cats.data.OptionT[cats.Eq,Err] = OptionT(cats.kernel.Eq$$anon$103@792e83c6)

optionTEqErr.narrow[NotFound]
// res7: cats.data.OptionT[cats.Eq,NotFound] = OptionT(cats.kernel.Eq$$anon$103@792e83c6)

Back to that compile error…

Let’s return to that covariant type A occurs in invariant position compile error that was triggered when we tried to declare final case class MyOptionT[F[_], +A](value: F[Option[A]]). Why did this happen?

Pretend for a minute that the Scala compiler did allow us to do this. We could then write:

val eqOptionNotFound: Eq[Option[NotFound]] = Eq.instance[Option[NotFound]]{
  case (None, None) => true
  case (None, Some(_)) => false
  case (Some(_), None) => false
  case (Some(x), Some(y)) => x.id === y.id
}
val optionTEqNotFound: MyOptionT[Eq, NotFound] = MyOptionT(eqOptionNotfound) // only allowed in our pretend world

val optionTEqErr: MyOptionT[Eq, Err] = optionTEqNotFound // only allowed in our pretend world

Because MyOptionT would be covariant in A, this MyOptionT[Eq, NotFound] could be treated as a MyOptionT[Eq, Err]. That is, it would have a value that is an Eq[Option[Err]]. But if you look at how we implemented our equality check, it’s taking two NotFound instances and comparing their id fields. For a general Err, we have no guarantee that it will be a NotFound and that it will have an id field. We can’t treat an Eq[Option[NotFound]] as an Eq[Option[Err]], because Eq is contravariant and not covariant in nature. The covariant type A occurs in invariant position message that the scala compiler gave us was the compiler correctly identifying that our code was unsound.

Conclusion

There are some use-cases in which having monad transforers such as OptionT and EitherT be defined as covariant would be convenient, such as helping to nudge type inference in the right direction. However, if Cats were to define these types as covariant, it would eliminate the possibility of using them with non-covariant types. Forcing covariance in Cats would be overly restrictive, since third-party libraries might not declare types as covariant, and monad transformers can be useful for invariant a contravariant types. Luckily, methods such as widen, leftWiden, and narrow provide concise solutions to turning a monad transformer (or other type that forms a functor) into the type that you need.

Licensing

Unless otherwise noted, all content is licensed under a Creative Commons Attribution 3.0 Unported License.

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