Frequently Asked Questions

• FAQ
• What imports do I need?
• I am new to pure functional programming, what quick wins can I get from Cats?
• What is the difference between Cats and Scalaz?
• Where is right-biased Either?
• Why is the compiler having trouble with types with more than one type parameter?
• Why is some example code not compiling for me?
• Why can't the compiler find implicit instances for Future?
• Where are implicit instances for Seq?
• How can I turn my List of <something> into a <something> of a list?
• Where is ListT?
• Where are Applicatives for monad transformers?
• Where is IO/Task?
• What do types like ? and λ mean?
• What is tailRecM?
• What does this symbol mean?
• How can I test instances against their type classes' laws?
• How can I help?
• How to try Cats in a REPL?
• Why aren't monad transformers like OptionT and EitherT covariant like Option and Either?

What imports do I need?

The easiest approach to Cats imports is to import everything that's commonly needed:

import cats._
import cats.data._
import cats.syntax.all._

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.

I am new to pure functional programming, what quick wins can I get from Cats?

Please refer to the jump start guide.

What is the difference between Cats and Scalaz?

Cats and Scalaz have the same goal: to facilitate pure functional programming in Scala applications. However the underlying core strategy is different; Scalaz took the approach of trying to provide a single batteries-included standard library for FP that powers the Scala applications. Cats, on the other hand, aims to help build an ecosystem of pure FP libraries by providing a solid and stable foundation; these libraries can have their own styles and personalities, competing with each other, while at the same time playing nice. It is through this ecosystem of FP libraries (cats included) that Scala applications can be powered with "FP awesome-ness" and beyond by picking whatever best fit their needs.

Based on this core strategy, Cats takes a modular approach and focuses on providing core, binary compatible, approachable and efficient abstractions. It provides a welcoming and supportive environment for the user community governed by the Scala Code of Conduct. It also takes great effort in supplying a comprehensive and beginner-friendly documentation.

Where is right-biased Either?

Up through Cats 0.7.x we had cats.data.Xor, 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.syntax._, which will also bring in syntactic enrichment and instances for other standard library types, or you can import only the Either enrichment with cats.syntax.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, due to Either having covariant type parameters.

Similarly, cats.data.XorT has been replaced with cats.data.EitherT, 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 scala.concurrent.ExecutionContext.Implicits.global, but note that you may want to use a different execution context for your production application.

Where are implicit instances for Seq?

As of cats-2.3, instances for collection.immutable.Seq are provided by cats. Mind that, up to scala-2.12, Seq was an alias for collection.Seq and lawful instances can't be provided for it due to its potential mutability. In scala-2.13, Seq was changed to collection.immutable.Seq which greatly improves Seq's interoperability with cats.

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

l.map(_.map(_ + 1))
// res1: Option[List[Int]] = Some(value = List(2, 3, 4, 5, 6))
l.exists(_.exists(isEven))
// 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.data.Nested
import cats.syntax.all._

val nl = Nested(l)
// nl: Nested[Option, List, Int] = Nested(
//   value = Some(value = List(1, 2, 3, 4, 5))
// )
nl.map(_ + 1)
// res3: Nested[Option, List, Int] = Nested(
//   value = Some(value = List(2, 3, 4, 5, 6))
// )
nl.exists(isEven)
// res4: Boolean = true

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

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

nl.traverse(even)
// res5: ErrorsOr[Nested[Option, List, Int]] = Invalid(
//   e = NonEmptyList(head = "1 is odd", tail = List("3 is odd", "5 is odd"))
// )

Where are Applicatives for monad transformers?

An Applicative instance for OptionT[F, *]/EitherT[F, E, *], built without a corresponding Monad instance for F, would be unlawful, so it's not included. See the guidelines for a more detailed explanation.

As an alternative, using .toNested on the monad transformer is recommended, although its ap will still be inconsistent with the Monad instance's.`.

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. In some languages and frameworks, such a type also serves as the primary abstraction through which parallelism is achieved. Nearly every real-world purely functional application or service is going to require such a data type, and this gives rise to an obvious question: why doesn't Cats include such a type?

The answer is that Cats does include an IO, it just isn't included in the core library. The decision was made to split IO away from cats-core and (indeed the whole Cats release cycle!) in order to make it easier to ensure modular versioning and compatibility across the ecosystem. The cats-effect project defines a type, cats.effect.IO, which is intended to be a very minimal, very performant data type for managing synchronous and asynchronous side-effects, integrated into the Cats ecosystem.

However, we acknowledge that this type may not meet everyone's needs. The cats-effect project characterizes the space of side-effect-capturing data types with a set of typeclasses (deriving from cats.Monad), and so all such data types are, broadly-speaking, mutually compatible and interchangeable in many generic contexts. For example, Monix provides support for IO, concurrency, and streaming and integrates with the cats-effect type classes.

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 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 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.

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 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] = fa.map(f)
  def flatMap[A, B](fa: Option[A])(f: A => Option[B]): Option[B] = fa.flatMap(f)

  @tailrec
  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].

What does this symbol mean?

Below is a list of symbols used in Cats.

The ~>, ⊥, ⊤, :<: and :≺: symbols can be imported with import cats._.

All other symbols can be imported with import cats.syntax.all._

How can I test instances against their type classes' laws?

You can find more information here.

How can I help?

The Сats community welcomes and encourages contributions, even if you are completely new to Сats 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.
• 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 Сats, you may want to look for items with the low-hanging-fruit label.

See the contributing guide for more information.

How to try Cats in a REPL?

The easiest way is probably using Ammonite-REPL. Install it following the instructions there. Then in the amm console you can type in

// interp.configureCompiler(_.settings.YpartialUnification.value = true) // If using scala 2.11 or 2.12
import $ivy.`org.typelevel::cats-core:2.1.1`, cats._, cats.data._, cats.implicits._

Or if you want, you can add these lines to ~/.ammonite/predef.sc so that they are enabled every ammonite session.

Why aren't monad transformers like OptionT and EitherT covariant like Option and Either?

Please see Variance of Monad Transformers on the Typelevel blog.