Clean Up Predicate A Bit (#269)

core/src/main/scala/cats/collections/Predicate.scala

@@ -16,7 +16,7 @@ abstract class Predicate[-A] extends scala.Function1[A, Boolean] { self =>
   /**
    * returns a predicate which is the union of this predicate and another
    */
-  def |[B <: A](other: Predicate[B]): Predicate[B] = Predicate(a => apply(a) || other(a))
+  def |[B <: A](other: Predicate[B]): Predicate[B] = self union other
 
   /**
    * returns a predicate which is the intersection of this predicate and another
@@ -26,7 +26,7 @@ abstract class Predicate[-A] extends scala.Function1[A, Boolean] { self =>
   /**
    * returns a predicate which is the intersection of this predicate and another
    */
-  def &[B <: A](other: Predicate[B]): Predicate[B] = Predicate(a => apply(a) && other(a))
+  def &[B <: A](other: Predicate[B]): Predicate[B] = self intersection other
 
   /**
    * Returns true if the value satisfies the predicate.
@@ -41,7 +41,7 @@ abstract class Predicate[-A] extends scala.Function1[A, Boolean] { self =>
   /**
    * Returns the predicate which is the the difference of another predicate removed from this predicate
    */
-  def -[B <: A](remove: Predicate[B]): Predicate[B] = Predicate(a => apply(a) && !remove(a))
+  def -[B <: A](remove: Predicate[B]): Predicate[B] = self diff remove
 
   /**
    * Return the opposite predicate
@@ -63,12 +63,20 @@ object Predicate extends PredicateInstances {
 }
 
 trait PredicateInstances {
+  implicit def predicateContravariantMonoidal: ContravariantMonoidal[Predicate] = new ContravariantMonoidal[Predicate] {
+    override def contramap[A, B](fb: Predicate[A])(f: B => A): Predicate[B] =
+      Predicate(f andThen fb.apply)
+    override def product[A, B](fa: Predicate[A], fb: Predicate[B]): Predicate[(A, B)] =
+      Predicate(v => fa(v._1) || fb(v._2))
+    override def unit: Predicate[Unit] = Predicate.empty
+  }
+
   implicit def predicateMonoid[A]: Monoid[Predicate[A]] = new Monoid[Predicate[A]] {
     override def empty: Predicate[A] = Predicate.empty
     override def combine(l: Predicate[A], r: Predicate[A]): Predicate[A] = l union r
   }
 
-  implicit val predicateInstance: MonoidK[Predicate] = new MonoidK[Predicate] {
+  implicit val predicateMonoidK: MonoidK[Predicate] = new MonoidK[Predicate] {
     override def empty[A]: Predicate[A] = Predicate.empty
     override def combineK[A](l: Predicate[A], r: Predicate[A]): Predicate[A] = l union r
   }

tests/src/test/scala/cats/collections/PredicateSpec.scala

@@ -1,13 +1,26 @@
 package cats.collections
 package tests
 
-import cats.laws.discipline.SerializableTests
+import cats.collections.arbitrary.predicate._
+import cats.laws.discipline.{ContravariantMonoidalTests, SerializableTests}
 import cats._
+import cats.instances.int._
 import cats.tests.CatsSuite
 
 class PredicateSpec extends CatsSuite {
-
+  checkAll("Monoid[Predicate[Int]]", SerializableTests.serializable(Monoid[Predicate[Int]]))
   checkAll("MonoidK[Predicate]", SerializableTests.serializable(MonoidK[Predicate]))
+  checkAll("Serializable[ContravariantMonoidal[Predicate]]", SerializableTests.serializable(ContravariantMonoidal[Predicate]))
+
+  {
+    implicit val eqForPredicateInt: Eq[Predicate[Int]] = new Eq[Predicate[Int]] {
+      override def eqv(x: Predicate[Int], y: Predicate[Int]): Boolean = x(0) === y(0)
+    }
+    implicit val eqForPredicateTripleInt: Eq[Predicate[(Int, Int, Int)]] = new Eq[Predicate[(Int, Int, Int)]] {
+      override def eqv(x: Predicate[(Int, Int, Int)], y: Predicate[(Int, Int, Int)]): Boolean = x((0, 0, 0)) === y((0, 0, 0))
+    }
+    checkAll("ContravariantMonoidal[Predicate]", ContravariantMonoidalTests[Predicate].contravariantMonoidal[Int, Int, Int])
+  }
 
   test("intersection works")(
     forAll { (as: List[Int], bs: List[Int]) =>