Frequently Asked Questions


What imports do I need?

The easiest approach to cats imports is to import everything that’s commonly needed:

import cats._
import cats.implicits._

This should be all that you need, but if you’d like to learn more about the details of imports than you can check out the import guide.

Where is right-biased Either?

Up through Cats 0.7.x we had, which was effectively scala.util.Either, but right-biased by default and with a bunch of useful combinators around it. In Scala 2.12.x Either became right-biased so we revisited the use of Xor and decided that in the interest of interoperability, we would remove Xor in the Cats 0.8.0 release and fill in the gaps in the scala.util.Either API via syntax enrichment.

This syntax and the type class instances for Either can be imported using cats.implicits._, which will also bring in syntactic enrichment and instances for other standard library types, or you can import them individually with cats.syntax.either._ and cats.instances.either._.

There are a few minor mismatches between Xor and Either. For example, in some cases you may need to specify a type parameter for an enrichment method on Either (such as leftMap) even though it was properly inferred for Xor. See the Either section of this guide for more information about these issues.

Similarly, has been replaced with, although since this is a type defined in Cats, you don’t need to import syntax or instances for it (although you may need imports for the underlying monad).

Why is the compiler having trouble with types with more than one type parameter?

When you encounter a situation where the same code works fine with a type with one type parameter, e.g. List[A], but doesn’t work with types with more than one, e.g. Either[A, B], you probably hit SI-2712. Without going into the details, it’s highly recommended to enable a partial SI-2712 fix in your project. The easiest way to achieve that is through this sbt plugin. Cats used to provide mitigation to this issue semi-transparently, but given the fact that the fix is now mainstream, we decided to drop that mitigation machinery in favor of reducing the complexity. See this issue for details.

Why is some example code not compiling for me?

A portion of example code requires either the Kind-projector compiler plugin or partial unification turned on in scalac. The easiest way to turn partial unification on is through this sbt plugin.

Why can’t the compiler find implicit instances for Future?

If you have already followed the imports advice but are still getting error messages like could not find implicit value for parameter e: cats.Monad[scala.concurrent.Future] or value |+| is not a member of scala.concurrent.Future[Int], then make sure that you have an implicit scala.concurrent.ExecutionContext in scope. The easiest way to do this is to import, but note that you may want to use a different execution context for your production application.

How can I turn my List of <something> into a <something> of a list?

It’s really common to have a List of values with types like Option, Either, or Validated that you would like to turn “inside out” into an Option (or Either or Validated) of a List. The sequence and traverse methods are really handy for this. You can read more about them in the Traverse documentation.

Where is ListT?

There are monad transformers for various types, such as OptionT, so people often wonder why there isn’t a ListT. For example, in the following example, people might reach for ListT to simplify making nested map and exists calls:

val l: Option[List[Int]] = Some(List(1, 2, 3, 4, 5))

def isEven(i: Int): Boolean = i % 2 == 0 + 1))
// res1: Option[List[Int]] = Some(List(2, 3, 4, 5, 6))

// res2: Boolean = true

A naive implementation of ListT suffers from associativity issues; see this gist for an example. It’s possible to create a ListT that doesn’t have these issues, but it tends to be pretty inefficient. For many use-cases, Nested can be used to achieve the desired results.

Here is how we could achieve the effect of the previous example using Nested:

import cats.implicits._
val nl = Nested(l)
// nl:[Option,List,Int] = Nested(Some(List(1, 2, 3, 4, 5))) + 1)
// res3:[Option,List,Int] = Nested(Some(List(2, 3, 4, 5, 6)))

// res4: Boolean = true

We can even perform more complicated operations, such as a traverse of the nested structure:

type ErrorsOr[A] = ValidatedNel[String, A]
def even(i: Int): ErrorsOr[Int] = if (i % 2 == 0) i.validNel else s"$i is odd".invalidNel

// res6: ErrorsOr[[Option,List,Int]] = Invalid(NonEmptyList(1 is odd, 3 is odd, 5 is odd))

Where is IO/Task?

In purely functional programming, a monadic IO or Task type is often used to handle side effects such as file/network IO. There have been several GitHub issues/PRs about this and many more Gitter/IRL conversations, but they all seem to arrive at the conclusion that there isn’t a clear canonical IO or Task that serves everyones’ needs. Some of the questions that come up are:

  • Should tasks be interruptible?
  • Should there be a single Task that subsumes IO and adds support for asynchrony and concurrency, or should there be separate IO and Task types?
  • How should concurrency be managed/configured?
  • Is scala.js supported?
  • Should you be able to block a thread waiting for the result of a task? This is really convenient for tests, but it isn’t really compatible with a JavaScript runtime and therefore is an issue for scala.js.

For some use-cases, a very simple IO is the best answer, as it avoids a lot of the overhead and complexity of other solutions. However, other use-cases require low-level concurrency control with asynchrony, resource management, and stack-safety. Considering all of the competing concerns, Cats has opted to not implement its own IO/Task types and instead encourage users to use a separate library that best serves their use-case.

Here are a couple libraries with Task implementations that you may find useful (in no particular order):

  • Monix - Asynchronous Programming for Scala and Scala.js.
    • The monix-eval module provides a full-featured Task that is both cancellable and Scala.js-compatible.
  • fs2 - Compositional, streaming I/O library for Scala
    • fs2 provides a Task that is a convenient option if you are already using fs2. fs2’s Task is also Scala.js-compatible.

It may be worth keeping in mind that IO and Task are pretty blunt instruments (they are essentially the Any of side effect management), and you may want to narrow the scope of your effects throughout most of your application. The free monad documentation describes a way to abstractly define controlled effects and interpret them into a type such as IO or Task (or even simply Try, Future, or Id) as late as possible. As more of your code becomes pure through these controlled effects the less it matters which type you end up choosing to represent your side effects.

What does @typeclass mean?

Cats defines and implements numerous type classes. Unfortunately, encoding these type classes in Scala can incur a large amount of boilerplate. To address this, Simulacrum introduces @typeclass, a macro annotation which generates a lot of this boilerplate. This elevates type classes to a first class construct and increases the legibility and maintainability of the code. Use of simulacrum also ensures consistency in how the type classes are encoded across a project. Cats uses simulacrum wherever possible to encode type classes, and you can read more about it at the project page.

Note that the one area where simulacrum is intentionally not used is in the cats-kernel module. The cats-kernel module is intended to be a shared dependency for a number of projects, and as such, it is important that it is both lightweight and very stable from a binary compatibility perspective. At some point there may be a transition from simulacrum to typeclassic, and the binary compatibility of moving between simulacrum and typeclassic is unclear at this point. Avoiding the dependency on simulacrum in cats-kernel, provides insulation against any potential binary compatibility problems in such a transition.

What do types like ? and λ mean?

Cats defines a wealth of type classes and type class instances. For a number of the type class and instance combinations, there is a mismatch between the type parameter requirements of the type class and the type parameter requirements of the data type for which the instance is being defined. For example, the Either data type is a type constructor with two type parameters. We would like to be able to define a Monad for Either, but the Monad type class operates on type constructors having only one type parameter.

Enter type lambdas! Type lambdas provide a mechanism to allow one or more of the type parameters for a particular type constructor to be fixed. In the case of Either then, when defining a Monad for Either, we want to fix one of the type parameters at the point where a Monad instance is summoned, so that the type parameters line up. As Either is right biased, a type lambda can be used to fix the left type parameter and allow the right type parameter to continue to vary when Either is treated as a Monad. The right biased nature of Either is discussed further in the Either documentation.

Enter kind-projector! kind-projector is a compiler plugin which provides a convenient syntax for dealing with type lambdas. The symbols ? and λ are treated specially by kind-projector, and expanded into the more verbose definitions that would be required were it not to be used. You can read more about kind-projector at the project page.

What does macro Ops do? What is cats.macros.Ops?

macro Ops invokes the Machinist Ops macro, and is used in cats in a number of places to enrich types with operations with the minimal possible cost when those operations are called in code. Machinist supports an extension mechanism where users of the macro can provide a mapping between symbolic operator names and method names. The cats.macros.Ops class uses this extension mechanism to supply the set of mappings that the cats project is interested in.

More about the history of machinist and how it works can be discovered at the project page, or this article on the typelevel blog.

What is tailRecM?

The FlatMap type class has a tailRecM method with the following signature:

def tailRecM[A, B](a: A)(f: A => F[Either[A, B]]): F[B]

When you are defining a FlatMap instance, its tailRecM implementation must have two properties in order for the instance to be considered lawful. The first property is that tailRecM must return the same result that you would get if you recursively called flatMap until you got a Right value (assuming you had unlimited stack space—we’ll get to that in a moment). In other words, it must give the same result as this implementation:

trait Monad[F[_]] {
  def pure[A](x: A): F[A] = ???
  def flatMap[A, B](fa: F[A])(f: A => F[B]): F[B] = ???

  def tailRecM[A, B](a: A)(f: A => F[Either[A, B]]): F[B] =
    flatMap(f(a)) {
      case Right(b) => pure(b)
      case Left(nextA) => tailRecM(nextA)(f)

The reason we can’t simply use this implementation for all type constructors (and the reason that tailRecM is useful at all) is that for many monadic types, recursively flatMap-ing in this way will quickly exhaust the stack.

Option is one example of a monadic type whose flatMap consumes stack in such a way that nesting flatMap calls deeply enough (usually around a couple thousand levels) will result in a stack overflow. We can provide a stack-safe tailRecM implementation for Option, though:

import cats.FlatMap
import scala.annotation.tailrec

implicit val optionFlatMap: FlatMap[Option] = new FlatMap[Option] {
  def map[A, B](fa: Option[A])(f: A => B): Option[B] =
  def flatMap[A, B](fa: Option[A])(f: A => Option[B]): Option[B] = fa.flatMap(f)

  def tailRecM[A, B](a: A)(f: A => Option[Either[A, B]]): Option[B] = f(a) match {
    case None => None
    case Some(Left(a1)) => tailRecM(a1)(f)
    case Some(Right(b)) => Some(b)

Now we don’t have to worry about overflowing the stack, no matter how many times we have to call tailRecM before we get a Right.

This is useful because any operation that you would write using recursive flatMaps can be rewritten to use tailRecM, and if the FlatMap instance for your type constructor is lawful, you don’t have to worry about stack safety.

The downside is that how you write a lawful tailRecM for your type constructor may not always be obvious. For some type constructors, such as Future, recursively flatMap-ing is already safe, and the first simple implementation above will be lawful. For types like Option and Try, you’ll need to arrange the recursion in such a way that the tailRecM calls are tail calls (which you can confirm with Scala’s tailrec annotation). Collection types require yet another approach (see for example the implementation for List).

If you’re having trouble figuring out how to implement tailRecM lawfully, you can try to find an instance in Cats itself for a type that is semantically similar to yours (all of the FlatMap instances provided by Cats have lawful, stack-safe tailRecM implementations).

In some cases you may decide that providing a lawful tailRecM may be impractical or even impossible (if so we’d like to hear about it). For these cases we provide a way of testing all of the monad laws except for the stack safety of tailRecM: just replace MonadTests[F].monad[A, B, C] in your tests with MonadTests[F].stackUnsafeMonad[A, B, C].

How can I help?

The cats community welcomes and encourages contributions, even if you are completely new to cats and functional programming. Here are a few ways to help out:

  • Find an undocumented method and write a ScalaDoc entry for it. See Arrow.scala for some examples of ScalaDoc entries that use sbt-doctest.
  • Look at the code coverage report, find some untested code, and write a test for it. Even simple helper methods and syntax enrichment should be tested.
  • Find an open issue, leave a comment on it to let people know you are working on it, and submit a pull request. If you are new to cats, you may want to look for items with the low-hanging-fruit label.

See the contributing guide for more information.