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

import cats._
import cats.implicits._

The cats._ import brings in quite a few type classes (similar to interfaces) such as Monad, Semigroup, and Foldable. Instead of the entire cats package, you can import only the types that you need, for example:

import cats.Monad
import cats.Semigroup
import cats.Foldable

The, import brings in data structures such as Validated and State. Instead of the entire package, you can import only the types that you need, for example:


The cats.implicits._ import does a couple of things. Firstly, it brings in implicit type class instances for standard library types - so after this import you will have Monad[List] and Semigroup[Int] instances in implicit scope. Secondly, it adds syntax enrichment onto certain types to provide some handy methods such as right-biased Either combinators:

// Сats adds right-biased combinators to the standard library's Either
val e: Either[String, Int] = Right(3)
// e: Either[String, Int] = Right(3) + 1)
// res0: Either[String, Int] = Right(4)

// cats adds an orEmpty method to the standard library's Option
val o: Option[String] = None
// o: Option[String] = None
// res1: String = ""

If you’d like to import à la carte, you can do so, by importing from cats.instances for the type class instances and cats.syntax for syntax enrichment. For example, if you’d like to import the Monoid instance for String and the corresponding syntax:

import cats.instances.string._

"Hello, " |+| "World!"
// res2: String = "Hello, World!"

The first import pulls the Semigroup instance for String into the scope, while the second import adds the |+| syntax.

You can also import all syntax or all instances by importing cats.syntax.all._ or cats.instances.all._ respectively.

For data types included in cats (i.e. data structure from the package), all type class instances are bundled with their implementation and therefore do not need to be imported separately. For example, if we wanted to import NonEmptyList from the package and use its SemigroupK instance, we would not need to specifically import the instance:


NonEmptyList.of(1,2) <+> NonEmptyList.of(3,4)
// res3: NonEmptyList[Int] = NonEmptyList(1, List(2, 3, 4))

Note: Beware that if you import a type class instance or its syntax twice, you will receive conflicting implicits with a less than helpful error message. This usually happens when importing different type classes in the same hierarchy or when importing syntax enrichment for all type classes using cats.syntax.all._ or cats.implicits._ together with a more specific import like cats.syntax.option._ or cats.instances.either._. Below is an example of this phenomenon:

import cats.instances.all._
val x = -2 |+| 1

// now we also need access to isEmpty from Monoid
import cats.syntax.monoid._
(x |+| 1).isEmpty //error: value |+| is not a member of Int

Compilation fails on the second invocation of |+| because we now have conflicting implicits from Monoid and Semigroup.