Arrow 

As we saw, an arrow (or morphism) is a mapping between a domain and a codomain. Another way of thinking about it, is that its an abstract notion for things that behave like functions.

In Cats, an Arrow instance is provided for Function1[A, B], Kleisli[F[_], A, B], and Cokleisli[F[_], A, B].

Here’s the typeclass contract for Arrow:

package cats
package arrow

import cats.functor.Strong
import simulacrum.typeclass

@typeclass trait Arrow[F[_, _]] extends Split[F] with Strong[F] with Category[F] { self =>

  /**
   * Lift a function into the context of an Arrow
   */
  def lift[A, B](f: A => B): F[A, B]

  ....
}

Category 

Here’s the typeclass contract for Category:

package cats
package arrow

import simulacrum.typeclass

/**
 * Must obey the laws defined in cats.laws.CategoryLaws.
 */
@typeclass trait Category[F[_, _]] extends Compose[F] { self =>

  def id[A]: F[A, A]

  ....
}

Compose 

Here’s the typeclass contract for Compose:

package cats
package arrow

import simulacrum.typeclass

/**
 * Must obey the laws defined in cats.laws.ComposeLaws.
 */
@typeclass trait Compose[F[_, _]] { self =>

  @simulacrum.op("<<<", alias = true)
  def compose[A, B, C](f: F[B, C], g: F[A, B]): F[A, C]

  @simulacrum.op(">>>", alias = true)
  def andThen[A, B, C](f: F[A, B], g: F[B, C]): F[A, C] =
    compose(g, f)

  ....
}

This enables two operators <<< and >>>.

scala> import cats._, cats.data._, cats.implicits._
import cats._
import cats.data._
import cats.implicits._

scala> val f = (_:Int) + 1
f: Int => Int = <function1>

scala> val g = (_:Int) * 100
g: Int => Int = <function1>

scala> (f >>> g)(2)
res0: Int = 300

scala> (f <<< g)(2)
res1: Int = 201

Strong 

Let’s read Haskell’s Arrow tutorial:

First and second make a new arrow out of an existing arrow. They perform a transformation (given by their argument) on either the first or the second item of a pair. These definitions are arrow-specific.

Here’s Cat’s Strong:

package cats
package functor

import simulacrum.typeclass

/**
 * Must obey the laws defined in cats.laws.StrongLaws.
 */
@typeclass trait Strong[F[_, _]] extends Profunctor[F] {

  /**
   * Create a new `F` that takes two inputs, but only modifies the first input
   */
  def first[A, B, C](fa: F[A, B]): F[(A, C), (B, C)]

  /**
   * Create a new `F` that takes two inputs, but only modifies the second input
   */
  def second[A, B, C](fa: F[A, B]): F[(C, A), (C, B)]
}

This enables two methods first[C] and second[C].

scala> val f_first = f.first[Int]
f_first: ((Int, Int)) => (Int, Int) = <function1>

scala> f_first((1, 1))
res2: (Int, Int) = (2,1)

scala> val f_second = f.second[Int]
f_second: ((Int, Int)) => (Int, Int) = <function1>

scala> f_second((1, 1))
res3: (Int, Int) = (1,2)

Given that f here is a function to add one, I think it’s clear what f_first and f_second are doing.

Split 

(***) combines two arrows into a new arrow by running the two arrows on a pair of values (one arrow on the first item of the pair and one arrow on the second item of the pair).

This is called split in Cats.

package cats
package arrow

import simulacrum.typeclass

@typeclass trait Split[F[_, _]] extends Compose[F] { self =>

  /**
   * Create a new `F` that splits its input between `f` and `g`
   * and combines the output of each.
   */
  def split[A, B, C, D](f: F[A, B], g: F[C, D]): F[(A, C), (B, D)]
}

We can use it as split operator:

scala> (f split g)((1, 1))
res4: (Int, Int) = (2,100)