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cats-docs/cats/Comonad

Comonad

cats.Comonad
See theComonad companion object
trait Comonad[F[_]] extends CoflatMap[F]

Comonad

Comonad is the dual of Monad. Whereas Monads allow for the composition of effectful functions, Comonads allow for composition of functions that extract the value from their context.

Must obey the laws defined in cats.laws.ComonadLaws.

Attributes

Companion
object
Source
Comonad.scala
Graph
Supertypes
trait CoflatMap[F]
trait Functor[F]
trait Invariant[F]
trait Serializable
class Object
trait Matchable
class Any
Show all
Known subtypes
trait Bimonad[F]

Members list

Value members

Abstract methods

def extract[A](x: F[A]): A

extract is the dual of pure on Monad (via Applicative) and extracts the value from its context

extract is the dual of pure on Monad (via Applicative) and extracts the value from its context

Example:

scala> import cats.Id
scala> import cats.Comonad
scala> val id: Id[Int] = 3
scala> Comonad[Id].extract(id)
res0: cats.Id[Int] = 3

Attributes

Source
Comonad.scala

Inherited methods

def as[A, B](fa: F[A], b: B): F[B]

Replaces the A value in F[A] with the supplied value.

Replaces the A value in F[A] with the supplied value.

Example:

scala> import cats.Functor
scala> import cats.implicits.catsStdInstancesForList

scala> Functor[List].as(List(1,2,3), "hello")
res0: List[String] = List(hello, hello, hello)

Attributes

Inherited from:
Functor
Source
Functor.scala
def coflatMap[A, B](fa: F[A])(f: F[A] => B): F[B]

coflatMap is the dual of flatMap on FlatMap.

coflatMap is the dual of flatMap on FlatMap. It applies a value in a context to a function that takes a value in a context and returns a normal value.

Example:

scala> import cats.syntax.all._
scala> import cats.CoflatMap
scala> val fa = Some(3)
scala> def f(a: Option[Int]): Int = a match {
    | case Some(x) => 2 * x
    | case None => 0 }
scala> CoflatMap[Option].coflatMap(fa)(f)
res0: Option[Int] = Some(6)

Attributes

Inherited from:
CoflatMap
Source
CoflatMap.scala
def coflatten[A](fa: F[A]): F[F[A]]

coflatten is the dual of flatten on FlatMap.

coflatten is the dual of flatten on FlatMap. Whereas flatten removes a layer of F, coflatten adds a layer of F

Example:

scala> import cats.syntax.all._
scala> import cats.CoflatMap
scala> val fa = Some(3)
fa: Option[Int] = Some(3)
scala> CoflatMap[Option].coflatten(fa)
res0: Option[Option[Int]] = Some(Some(3))

Attributes

Inherited from:
CoflatMap
Source
CoflatMap.scala
def compose[G[_] : Functor]: Functor[[α] =>> F[G[α]]]

Attributes

Inherited from:
Functor
Source
Functor.scala
def compose[G[_] : Invariant]: Invariant[[α] =>> F[G[α]]]

Compose Invariant F[_] and G[_] then produce Invariant[F[G[_]]] using their imap.

Compose Invariant F[_] and G[_] then produce Invariant[F[G[_]]] using their imap.

Example:

scala> import cats.syntax.all._
scala> import scala.concurrent.duration._

scala> val durSemigroupList: Semigroup[List[FiniteDuration]] =
    | Invariant[Semigroup].compose[List].imap(Semigroup[List[Long]])(Duration.fromNanos)(_.toNanos)
scala> durSemigroupList.combine(List(2.seconds, 3.seconds), List(4.seconds))
res1: List[FiniteDuration] = List(2 seconds, 3 seconds, 4 seconds)

Attributes

Inherited from:
Invariant
Source
Invariant.scala
def composeBifunctor[G[_, _] : Bifunctor]: Bifunctor[[α, β] =>> F[G[α, β]]]

Attributes

Inherited from:
Functor
Source
Functor.scala
override def composeContravariant[G[_] : Contravariant]: Contravariant[[α] =>> F[G[α]]]

Compose Invariant F[_] and Contravariant G[_] then produce Invariant[F[G[_]]] using F's imap and G's contramap.

Compose Invariant F[_] and Contravariant G[_] then produce Invariant[F[G[_]]] using F's imap and G's contramap.

Example:

scala> import cats.syntax.all._
scala> import scala.concurrent.duration._

scala> type ToInt[T] = T => Int
scala> val durSemigroupToInt: Semigroup[ToInt[FiniteDuration]] =
    | Invariant[Semigroup]
    |   .composeContravariant[ToInt]
    |   .imap(Semigroup[ToInt[Long]])(Duration.fromNanos)(_.toNanos)
// semantically equal to (2.seconds.toSeconds.toInt + 1) + (2.seconds.toSeconds.toInt * 2) = 7
scala> durSemigroupToInt.combine(_.toSeconds.toInt + 1, _.toSeconds.toInt * 2)(2.seconds)
res1: Int = 7

Attributes

Definition Classes
Inherited from:
Functor
Source
Functor.scala
def composeFunctor[G[_] : Functor]: Invariant[[α] =>> F[G[α]]]

Compose Invariant F[_] and Functor G[_] then produce Invariant[F[G[_]]] using F's imap and G's map.

Compose Invariant F[_] and Functor G[_] then produce Invariant[F[G[_]]] using F's imap and G's map.

Example:

scala> import cats.syntax.all._
scala> import scala.concurrent.duration._

scala> val durSemigroupList: Semigroup[List[FiniteDuration]] =
    | Invariant[Semigroup]
    |   .composeFunctor[List]
    |   .imap(Semigroup[List[Long]])(Duration.fromNanos)(_.toNanos)
scala> durSemigroupList.combine(List(2.seconds, 3.seconds), List(4.seconds))
res1: List[FiniteDuration] = List(2 seconds, 3 seconds, 4 seconds)

Attributes

Inherited from:
Invariant
Source
Invariant.scala
final def fmap[A, B](fa: F[A])(f: A => B): F[B]

Alias for map, since map can't be injected as syntax if the implementing type already had a built-in .map method.

Alias for map, since map can't be injected as syntax if the implementing type already had a built-in .map method.

Example:

scala> import cats.syntax.all._

scala> val m: Map[Int, String] = Map(1 -> "hi", 2 -> "there", 3 -> "you")

scala> m.fmap(_ ++ "!")
res0: Map[Int,String] = Map(1 -> hi!, 2 -> there!, 3 -> you!)

Attributes

Inherited from:
Functor
Source
Functor.scala
def fproduct[A, B](fa: F[A])(f: A => B): F[(A, B)]

Tuple the values in fa with the result of applying a function with the value

Tuple the values in fa with the result of applying a function with the value

Example:

scala> import cats.Functor
scala> import cats.implicits.catsStdInstancesForOption

scala> Functor[Option].fproduct(Option(42))(_.toString)
res0: Option[(Int, String)] = Some((42,42))

Attributes

Inherited from:
Functor
Source
Functor.scala
def fproductLeft[A, B](fa: F[A])(f: A => B): F[(B, A)]

Pair the result of function application with A.

Pair the result of function application with A.

Example:

scala> import cats.Functor
scala> import cats.implicits.catsStdInstancesForOption

scala> Functor[Option].fproductLeft(Option(42))(_.toString)
res0: Option[(String, Int)] = Some((42,42))

Attributes

Inherited from:
Functor
Source
Functor.scala
def ifF[A](fb: F[Boolean])(ifTrue: => A, ifFalse: => A): F[A]

Lifts if to Functor

Lifts if to Functor

Example:

scala> import cats.Functor
scala> import cats.implicits.catsStdInstancesForList

scala> Functor[List].ifF(List(true, false, false))(1, 0)
res0: List[Int] = List(1, 0, 0)

Attributes

Inherited from:
Functor
Source
Functor.scala
override def imap[A, B](fa: F[A])(f: A => B)(g: B => A): F[B]

Transform an F[A] into an F[B] by providing a transformation from A to B and one from B to A.

Transform an F[A] into an F[B] by providing a transformation from A to B and one from B to A.

Example:

scala> import cats.syntax.all._
scala> import scala.concurrent.duration._

scala> val durSemigroup: Semigroup[FiniteDuration] =
    | Invariant[Semigroup].imap(Semigroup[Long])(Duration.fromNanos)(_.toNanos)
scala> durSemigroup.combine(2.seconds, 3.seconds)
res1: FiniteDuration = 5 seconds

Attributes

Definition Classes
Inherited from:
Functor
Source
Functor.scala
def lift[A, B](f: A => B): F[A] => F[B]

Lift a function f to operate on Functors

Lift a function f to operate on Functors

Example:

scala> import cats.Functor
scala> import cats.implicits.catsStdInstancesForOption

scala> val o = Option(42)
scala> Functor[Option].lift((x: Int) => x + 10)(o)
res0: Option[Int] = Some(52)

Attributes

Inherited from:
Functor
Source
Functor.scala
def map[A, B](fa: F[A])(f: A => B): F[B]

Attributes

Inherited from:
Functor
Source
Functor.scala
def tupleLeft[A, B](fa: F[A], b: B): F[(B, A)]

Tuples the A value in F[A] with the supplied B value, with the B value on the left.

Tuples the A value in F[A] with the supplied B value, with the B value on the left.

Example:

scala> import scala.collection.immutable.Queue
scala> import cats.Functor
scala> import cats.implicits.catsStdInstancesForQueue

scala> Functor[Queue].tupleLeft(Queue("hello", "world"), 42)
res0: scala.collection.immutable.Queue[(Int, String)] = Queue((42,hello), (42,world))

Attributes

Inherited from:
Functor
Source
Functor.scala
def tupleRight[A, B](fa: F[A], b: B): F[(A, B)]

Tuples the A value in F[A] with the supplied B value, with the B value on the right.

Tuples the A value in F[A] with the supplied B value, with the B value on the right.

Example:

scala> import scala.collection.immutable.Queue
scala> import cats.Functor
scala> import cats.implicits.catsStdInstancesForQueue

scala> Functor[Queue].tupleRight(Queue("hello", "world"), 42)
res0: scala.collection.immutable.Queue[(String, Int)] = Queue((hello,42), (world,42))

Attributes

Inherited from:
Functor
Source
Functor.scala
def unzip[A, B](fab: F[(A, B)]): (F[A], F[B])

Un-zips an F[(A, B)] consisting of element pairs or Tuple2 into two separate F's tupled.

Un-zips an F[(A, B)] consisting of element pairs or Tuple2 into two separate F's tupled.

NOTE: Check for effect duplication, possibly memoize before

scala> import cats.Functor
scala> import cats.implicits.catsStdInstancesForList

scala> Functor[List].unzip(List((1,2), (3, 4)))
res0: (List[Int], List[Int]) = (List(1, 3),List(2, 4))

Attributes

Inherited from:
Functor
Source
Functor.scala
def void[A](fa: F[A]): F[Unit]

Empty the fa of the values, preserving the structure

Empty the fa of the values, preserving the structure

Example:

scala> import cats.Functor
scala> import cats.implicits.catsStdInstancesForList

scala> Functor[List].void(List(1,2,3))
res0: List[Unit] = List((), (), ())

Attributes

Inherited from:
Functor
Source
Functor.scala
def widen[A, B >: A](fa: F[A]): F[B]

Lifts natural subtyping covariance of covariant Functors.

Lifts natural subtyping covariance of covariant Functors.

NOTE: In certain (perhaps contrived) situations that rely on universal equality this can result in a ClassCastException, because it is implemented as a type cast. It could be implemented as map(identity), but according to the functor laws, that should be equal to fa, and a type cast is often much more performant. See this example of widen creating a ClassCastException.

Example:

scala> import cats.Functor
scala> import cats.implicits.catsStdInstancesForOption

scala> val s = Some(42)
scala> Functor[Option].widen(s)
res0: Option[Int] = Some(42)

Attributes

Inherited from:
Functor
Source
Functor.scala
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