Add PartialFunction instance for Profunctor typeclass (#3392)

* added PartialFunction instance for Profunctor typeclass

* updated doctest to use as extension method

* added entire arrow heirarchy for PartialFunction

* addressed review feedback, moved implicits to the top of the hierarchy (available to all)

* added back instance check when lifting a function to partialFunction

* addressed review feedback, have `val` as `def` instead

* updated to have PartialFunctionInstances mixed in with AllInstances; improved doctests with less imports now needed

core/src/main/scala-2.12/cats/instances/all.scala

@@ -43,6 +43,7 @@ trait AllInstances
     with TupleInstances
     with UUIDInstances
     with VectorInstances
+    with PartialFunctionInstances
 
 trait AllInstancesBinCompat0 extends FunctionInstancesBinCompat0 with Tuple2InstancesBinCompat0
 

core/src/main/scala-2.12/cats/instances/package.scala

@@ -16,6 +16,7 @@ package object instances {
   object float extends FloatInstances
   object finiteDuration extends CoreFiniteDurationInstances with FiniteDurationInstances
   object function extends FunctionInstances with FunctionInstancesBinCompat0
+  object partialFunction extends PartialFunctionInstances
   object future extends FutureInstances
   object int extends IntInstances
   object invariant extends InvariantMonoidalInstances

core/src/main/scala-2.13+/cats/instances/all.scala

@@ -45,6 +45,7 @@ trait AllInstances
     with TupleInstances
     with UUIDInstances
     with VectorInstances
+    with PartialFunctionInstances
 
 trait AllInstancesBinCompat0 extends FunctionInstancesBinCompat0 with Tuple2InstancesBinCompat0
 

core/src/main/scala-2.13+/cats/instances/package.scala

@@ -17,6 +17,7 @@ package object instances {
   object float extends FloatInstances
   object finiteDuration extends CoreFiniteDurationInstances with FiniteDurationInstances
   object function extends FunctionInstances with FunctionInstancesBinCompat0
+  object partialFunction extends PartialFunctionInstances
   object future extends FutureInstances
   object int extends IntInstances
   object invariant extends InvariantMonoidalInstances

core/src/main/scala/cats/Applicative.scala

@@ -202,7 +202,7 @@ object Applicative {
    * scala> import cats.Applicative.catsApplicativeForArrow
    * scala> val toLong: Int => Long = _.toLong
    * scala> val double: Int => Int = 2*_
-   * scala> val f: Int => (Long, Int) = catsApplicativeForArrow.product(toLong, double)
+   * scala> val f: Int => (Long, Int) = catsApplicativeForArrow[Function1, Int].product(toLong, double)
    * scala> f(3)
    * res0: (Long, Int) = (3,6)
    * }}}

core/src/main/scala/cats/arrow/Compose.scala

@@ -45,6 +45,9 @@ object Compose {
     cats.instances.function.catsStdInstancesForFunction1
   implicit def catsComposeForMap: Compose[Map] = cats.instances.map.catsStdComposeForMap
 
+  implicit def catsInstancesForPartialFunction: ArrowChoice[PartialFunction] with CommutativeArrow[PartialFunction] =
+    cats.instances.partialFunction.catsStdInstancesForPartialFunction
+
   /****************************************************************************/
   /* THE FOLLOWING CODE IS MANAGED BY SIMULACRUM; PLEASE DO NOT EDIT!!!!      */
   /****************************************************************************/

core/src/main/scala/cats/arrow/Profunctor.scala

@@ -47,6 +47,9 @@ object Profunctor {
   implicit def catsStrongForFunction1: Strong[Function1] =
     cats.instances.function.catsStdInstancesForFunction1
 
+  implicit def catsStrongForPartialFunction: Strong[PartialFunction] =
+    cats.instances.partialFunction.catsStdInstancesForPartialFunction
+
   /****************************************************************************/
   /* THE FOLLOWING CODE IS MANAGED BY SIMULACRUM; PLEASE DO NOT EDIT!!!!      */
   /****************************************************************************/

core/src/main/scala/cats/instances/function.scala

@@ -136,11 +136,10 @@ sealed private[instances] trait Function1Instances extends Function1Instances0 {
 
   implicit val catsStdInstancesForFunction1: ArrowChoice[Function1] with CommutativeArrow[Function1] =
     new ArrowChoice[Function1] with CommutativeArrow[Function1] {
-      def choose[A, B, C, D](f: A => C)(g: B => D): Either[A, B] => Either[C, D] =
-        _ match {
-          case Left(a)  => Left(f(a))
-          case Right(b) => Right(g(b))
-        }
+      def choose[A, B, C, D](f: A => C)(g: B => D): Either[A, B] => Either[C, D] = {
+        case Left(a)  => Left(f(a))
+        case Right(b) => Right(g(b))
+      }
 
       def lift[A, B](f: A => B): A => B = f
 

core/src/main/scala/cats/instances/partialFunction.scala

@@ -0,0 +1,64 @@
+package cats.instances
+import cats.arrow.{ArrowChoice, CommutativeArrow}
+
+trait PartialFunctionInstances {
+
+  implicit def catsStdInstancesForPartialFunction: ArrowChoice[PartialFunction] with CommutativeArrow[PartialFunction] =
+    PartialFunctionInstances.instance
+}
+
+private object PartialFunctionInstances {
+
+  private val instance: ArrowChoice[PartialFunction] with CommutativeArrow[PartialFunction] =
+    new ArrowChoice[PartialFunction] with CommutativeArrow[PartialFunction] {
+
+      /**
+       * {{{
+       * scala> import cats.arrow.Arrow
+       * scala> import cats.syntax.arrowChoice._
+       * scala> val toLong: PartialFunction[Int, Long] = Arrow[PartialFunction].lift(_.toLong)
+       * scala> val toDouble: PartialFunction[Float, Double] = Arrow[PartialFunction].lift(_.toDouble)
+       * scala> val f: PartialFunction[Either[Int, Float], Either[Long, Double]] = toLong +++ toDouble
+       * scala> f(Left(3))
+       * res0: Either[Long,Double] = Left(3)
+       * scala> f(Right(3))
+       * res1: Either[Long,Double] = Right(3.0)
+       * }}}
+       */
+      override def choose[A, B, C, D](
+        f: PartialFunction[A, C]
+      )(g: PartialFunction[B, D]): PartialFunction[Either[A, B], Either[C, D]] = {
+        case Left(a) if f.isDefinedAt(a)  => Left(f(a))
+        case Right(b) if g.isDefinedAt(b) => Right(g(b))
+      }
+
+      override def lift[A, B](f: A => B): PartialFunction[A, B] = { case a if a.isInstanceOf[A] => f(a) }
+
+      /**
+       * Create a new `F` that takes two inputs, but only modifies the first input
+       *
+       * Example:
+       * {{{
+       * scala> import cats.arrow.Arrow
+       * scala> val f: PartialFunction[Int, Int] = Arrow[PartialFunction].lift(_ * 2)
+       * scala> val fab = Arrow[PartialFunction].first[Int,Int,Int](f)
+       * scala> fab((2,3))
+       * res0: (Int, Int) = (4,3)
+       * }}}
+       */
+      override def first[A, B, C](fa: PartialFunction[A, B]): PartialFunction[(A, C), (B, C)] = {
+        case (a, c) if fa.isDefinedAt(a) => (fa(a), c)
+      }
+
+      override def split[A, B, C, D](
+        f: PartialFunction[A, B],
+        g: PartialFunction[C, D]
+      ): PartialFunction[(A, C), (B, D)] = {
+        case (a, c) if f.isDefinedAt(a) && g.isDefinedAt(c) => (f(a), g(c))
+      }
+
+      override def compose[A, B, C](f: PartialFunction[B, C], g: PartialFunction[A, B]): PartialFunction[A, C] = {
+        case a if g.isDefinedAt(a) && f.isDefinedAt(g(a)) => f(g(a))
+      }
+    }
+}

kernel/src/main/scala/cats/kernel/Eq.scala

@@ -1,12 +1,14 @@
 package cats.kernel
 
 import java.util.UUID
+
+import cats.kernel.compat.scalaVersionSpecific._
+
 import scala.collection.immutable.{BitSet, Queue, SortedMap, SortedSet}
 import scala.concurrent.duration.{Duration, FiniteDuration}
 import scala.math.Equiv
-import scala.{specialized => sp}
 import scala.util.{Failure, Success, Try}
-import compat.scalaVersionSpecific._
+import scala.{specialized => sp}
 
 /**
  * A type class used to determine equality between 2 instances of the same

laws/src/main/scala/cats/laws/discipline/Eq.scala

@@ -2,13 +2,10 @@ package cats
 package laws
 package discipline
 
-import cats.data.RepresentableStore
-import cats.Eq
-import cats.data.AndThen
+import cats.data.{AndThen, RepresentableStore}
 import cats.instances.boolean._
 import cats.instances.int._
 import cats.instances.string._
-import cats.instances.tuple._
 import cats.kernel._
 import cats.platform.Platform
 import cats.syntax.eq._
@@ -22,6 +19,13 @@ object eq {
   implicit def catsLawsEqForFn2[A, B, C](implicit ev: Eq[((A, B)) => C]): Eq[(A, B) => C] =
     Eq.by((_: (A, B) => C).tupled)
 
+  implicit def catsLawsEqForPartialFunctionExhaustive[A: ExhaustiveCheck, B: Eq]: Eq[PartialFunction[A, B]] =
+    Eq.instance((f, g) =>
+      ExhaustiveCheck[A].allValues
+        .filter(a => f.isDefinedAt(a) || g.isDefinedAt(a))
+        .forall(a => f.isDefinedAt(a) && g.isDefinedAt(a) && Eq[B].eqv(f(a), g(a)))
+    )
+
   implicit def catsLawsEqForAndThen[A, B](implicit eqAB: Eq[A => B]): Eq[AndThen[A, B]] =
     Eq.by[AndThen[A, B], A => B](identity)
 

tests/src/test/scala/cats/tests/PartialFunctionSuite.scala

@@ -0,0 +1,22 @@
+package cats.tests
+import cats.arrow.{ArrowChoice, CommutativeArrow}
+import cats.kernel.laws.discipline.SerializableTests
+import cats.laws.discipline.arbitrary._
+import cats.laws.discipline.eq._
+import cats.laws.discipline.{ArrowChoiceTests, CommutativeArrowTests, MiniInt}
+
+class PartialFunctionSuite extends CatsSuite {
+
+  checkAll("ArrowChoice[PartialFunction]", SerializableTests.serializable(ArrowChoice[PartialFunction]))
+
+  checkAll("PartialFunction",
+           ArrowChoiceTests[PartialFunction].arrowChoice[MiniInt, MiniInt, MiniInt, MiniInt, MiniInt, MiniInt]
+  )
+
+  checkAll("CommutativeArrow[PartialFunction]", SerializableTests.serializable(CommutativeArrow[PartialFunction]))
+  checkAll(
+    "PartialFunction",
+    CommutativeArrowTests[PartialFunction].commutativeArrow[MiniInt, MiniInt, MiniInt, MiniInt, MiniInt, MiniInt]
+  )
+
+}