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cats-docs/cats/Contravariant

Contravariant

cats.Contravariant
See theContravariant companion object
trait Contravariant[F[_]] extends Invariant[F]

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

Attributes

Companion
object
Source
Contravariant.scala
Graph
Supertypes
trait Invariant[F]
trait Serializable
class Object
trait Matchable
class Any
Known subtypes
Self type

Members list

Value members

Abstract methods

def contramap[A, B](fa: F[A])(f: B => A): F[B]

Attributes

Source
Contravariant.scala

Concrete methods

def compose[G[_] : Contravariant]: Functor[[α] =>> F[G[α]]]

Attributes

Source
Contravariant.scala
override def composeFunctor[G[_] : Functor]: Contravariant[[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

Definition Classes
Source
Contravariant.scala
override def imap[A, B](fa: F[A])(f: A => B)(fi: 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
Source
Contravariant.scala
def liftContravariant[A, B](f: A => B): F[B] => F[A]

Attributes

Source
Contravariant.scala
def narrow[A, B <: A](fa: F[A]): F[B]

Lifts natural subtyping contravariance of contravariant Functors.

Lifts natural subtyping contravariance of contravariant Functors. could be implemented as contramap(identity), but the Functor laws say this is equivalent

Attributes

Source
Contravariant.scala

Inherited methods

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 composeContravariant[G[_] : Contravariant]: Invariant[[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

Inherited from:
Invariant
Source
Invariant.scala
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