Make Predicate stack-safe using Eval

bench/src/main/scala/PredicateBenchmarks.scala

@@ -0,0 +1,25 @@
+package cats.collections
+package bench
+
+import org.openjdk.jmh.annotations.{Benchmark, Param, Scope, Setup, State}
+import cats._
+import cats.implicits._
+
+@State(Scope.Benchmark)
+class ChainedPredicateBench {
+  @Param(Array("10", "100", "1000", "10000"))
+  var n: Int = _
+
+  var pred: Predicate[Int] = _
+
+  @Setup
+  def setup: Unit = {
+    pred = Predicate(_ == 0)
+    pred = Iterator.iterate(pred.negate)(_ - pred).drop(n).next()
+  }
+
+  @Benchmark
+  def catsCollectionsPredicateUnravel: Unit = {
+    pred.contains(0)
+  }
+}

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

@@ -1,17 +1,40 @@
 package cats.collections
 
+import algebra.lattice.Bool
 import cats._
+import cats.data.Kleisli
 
 /**
  * An intensional set, which is a set which instead of enumerating its
  * elements as a extensional set does, it is defined by a predicate
  * which is a test for membership.
+ *
+ * All combinators in this class are implemented in a stack-safe way.
  */
-abstract class Predicate[-A] extends scala.Function1[A, Boolean] { self =>
+sealed abstract class Predicate[-A] extends scala.Function1[A, Boolean] { self =>
+  def run: Kleisli[Eval, A, Boolean]
+
+  /**
+   * Returns true if the value satisfies the predicate.
+   */
+  def apply(a: A): Boolean = run(a).value
+
   /**
    * returns a predicate which is the union of this predicate and another
    */
-  def union[B <: A](other: Predicate[B]): Predicate[B] = Predicate(a => apply(a) || other(a))
+  def union[B <: A](other: Predicate[B]): Predicate[B] =
+    self match {
+      case Predicate.Empty => other
+      case Predicate.Everything => self
+      case _ =>
+      other match {
+        case Predicate.Empty => self
+        case Predicate.Everything => other
+        case _ => Predicate.Lift {
+          self.run.flatMap(if (_) Predicate.True else other.run)
+        }
+      }
+    }
 
   /**
    * returns a predicate which is the union of this predicate and another
@@ -21,7 +44,19 @@ abstract class Predicate[-A] extends scala.Function1[A, Boolean] { self =>
   /**
    * returns a predicate which is the intersection of this predicate and another
    */
-  def intersection[B <: A](other: Predicate[B]): Predicate[B] = Predicate(a => apply(a) && other(a))
+  def intersection[B <: A](other: Predicate[B]): Predicate[B] =
+    self match {
+      case Predicate.Empty => self
+      case Predicate.Everything => other
+      case _ =>
+        other match {
+        case Predicate.Empty => other
+        case Predicate.Everything => self
+        case _ => Predicate.Lift {
+          self.run.flatMap(if (_) other.run else Predicate.False)
+        }
+      }
+    }
 
   /**
    * returns a predicate which is the intersection of this predicate and another
@@ -36,7 +71,7 @@ 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 diff[B <: A](remove: Predicate[B]): Predicate[B] = Predicate(a => apply(a) && !remove(a))
+  def diff[B <: A](remove: Predicate[B]): Predicate[B] = self intersection remove.negate
 
   /**
    * Returns the predicate which is the the difference of another predicate removed from this predicate
@@ -46,28 +81,77 @@ abstract class Predicate[-A] extends scala.Function1[A, Boolean] { self =>
   /**
    * Return the opposite predicate
    */
-  def negate: Predicate[A] = Predicate(a => !apply(a))
+  def negate: Predicate[A]
 
   /**
    * Return the opposite predicate
    */
   def unary_!(): Predicate[A] = negate
+
+  /**
+   * Compose the predicate with a function.
+   *
+   * A value is a member of the resulting predicate iff its image through f is a
+   * member of this predicate.
+   */
+  def contramap[B](f: B => A): Predicate[B]
+
+  /**
+   * Alias for contramap.
+   */
+  final override def compose[B](f: B => A): Predicate[B] = contramap(f)
 }
 
 object Predicate extends PredicateInstances {
-  def apply[A](f: A => Boolean): Predicate[A] = new Predicate[A] {
-    def apply(a: A) = f(a)
+  private val True = Kleisli.liftF(Eval.True)
+  private val False = Kleisli.liftF(Eval.False)
+
+  private[collections] case object Empty extends Predicate[Any] {
+    override def run: Kleisli[Eval, Any, Boolean] = Predicate.False
+    override def negate: Predicate[Any] = Everything
+    override def contramap[B](f: B => Any): Predicate[B] = this
   }
 
-  def empty: Predicate[Any] = apply(_ => false)
+  private[collections] case object Everything extends Predicate[Any] {
+    override def run: Kleisli[Eval, Any, Boolean] = Predicate.True
+    override def negate: Predicate[Any] = Empty
+    override def contramap[B](f: B => Any): Predicate[B] = this
+  }
+
+  private[collections] final case class Lift[A](run: Kleisli[Eval, A, Boolean]) extends Predicate[A] {
+    override def negate: Predicate[A] = Negate(this)
+    override def contramap[B](f: B => A): Predicate[B] = Lift(run.compose(b => Eval.now(f(b))))
+  }
+
+  private[collections] final case class Negate[A](negate: Predicate[A]) extends Predicate[A] {
+    override def run: Kleisli[Eval, A, Boolean] = negate.run.map(!_)
+    override def contramap[B](f: B => A): Predicate[B] = Negate(negate contramap f)
+  }
+
+  /**
+   * build a set from a membership function.
+  */
+  def apply[A](p: A => Boolean): Predicate[A] = Lift {
+    Kleisli(a => Eval.now(p(a)))
+  }
+
+  /**
+   * The empty set: a predicate that rejects all values.
+   */
+  def empty[A]: Predicate[A] = Empty
+
+  /**
+   * The set of everything: a predicate that accepts all values.
+   */
+  def everything[A]: Predicate[A] = Everything
 }
 
 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)
+      fb.contramap(f)
     override def product[A, B](fa: Predicate[A], fb: Predicate[B]): Predicate[(A, B)] =
-      Predicate(v => fa(v._1) || fb(v._2))
+      fa.contramap[(A,B)](_._1) union fb.contramap(_._2)
     override def unit: Predicate[Unit] = Predicate.empty
   }
 
@@ -76,6 +160,15 @@ trait PredicateInstances {
     override def combine(l: Predicate[A], r: Predicate[A]): Predicate[A] = l union r
   }
 
+  implicit def predicateBool[A]: Bool[Predicate[A]] = new Bool[Predicate[A]] {
+    override def one: Predicate[A] = Predicate.everything
+    override def zero: Predicate[A] = Predicate.empty
+    override def complement(x: Predicate[A]): Predicate[A] = x.negate
+    override def and(l: Predicate[A], r: Predicate[A]): Predicate[A] = l intersection r
+    override def or(l: Predicate[A], r: Predicate[A]): Predicate[A] = l union r
+
+  }
+
   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

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

@@ -409,6 +409,8 @@ object AvlSet extends AvlSetInstances {
   private[collections] case object BTNil extends AvlSet[Nothing] {
     override def isEmpty: Boolean = true
 
+    override def predicate(implicit order: cats.Order[Nothing]): Predicate[Nothing] = Predicate.empty
+
     def apply[A](): AvlSet[A] = this.asInstanceOf[AvlSet[A]]
 
     def unapply[A](a: AvlSet[A]): Boolean = a.isEmpty

scalacheck/src/main/scala/cats/collections/arbitrary/ArbitraryPredicate.scala

@@ -1,15 +1,13 @@
 package cats.collections
 package arbitrary
 
-import org.scalacheck.{Gen, Arbitrary}
-import cats.Order
+import org.scalacheck.{Gen, Cogen, Arbitrary}
 
 trait ArbitraryPredicate {
-  import set._
-  def predicateGen[A: Arbitrary: Order]: Gen[Predicate[A]] =
-    setGen.map(_.predicate)
+  import Gen._
+  def predicateGen[A: Arbitrary: Cogen]: Gen[Predicate[A]] =
+    oneOf(const(Predicate.empty), const(Predicate.everything), resultOf(Predicate(_: A => Boolean)))
 
-  implicit def predicateArbitrary[A: Arbitrary: Order]: Arbitrary[Predicate[A]] =
+  implicit def predicateArbitrary[A: Arbitrary: Cogen]: Arbitrary[Predicate[A]] =
     Arbitrary(predicateGen[A])
 }
-

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

@@ -1,6 +1,8 @@
 package cats.collections
+
 package tests
 
+import algebra.laws.LogicLaws
 import cats.collections.arbitrary.predicate._
 import cats.laws.discipline.{ContravariantMonoidalTests, SerializableTests}
 import cats._
@@ -22,6 +24,16 @@ class PredicateSpec extends CatsSuite {
     checkAll("ContravariantMonoidal[Predicate]", ContravariantMonoidalTests[Predicate].contravariantMonoidal[Int, Int, Int])
   }
 
+  {
+    implicit val eqForPredicateInt: Eq[Predicate[Int]] = new Eq[Predicate[Int]] {
+      val sample = -1 to 1 // need at least 2 elements to distinguish in-between values
+      override def eqv(x: Predicate[Int], y: Predicate[Int]): Boolean =
+        sample.forall(a => x(a) == y(a))
+    }
+
+    checkAll("Bool[Predicate[Int]]", LogicLaws[Predicate[Int]].bool)
+  }
+
   test("intersection works")(
     forAll { (as: List[Int], bs: List[Int]) =>
 
@@ -72,4 +84,31 @@ class PredicateSpec extends CatsSuite {
       bs.forall(b => (s1(b) != (as.contains(b) && (b % 2 != 0)))) should be(true)
 
     })
+
+  {
+    def testStackSafety(name: String, deepSet: => Predicate[Int]) =
+      test(name) {
+        noException should be thrownBy {
+          deepSet.contains(0)
+        }
+      }
+    val Depth = 200000
+    val NonZero = Predicate[Int](_ != 0)
+    testStackSafety("union is stack safe on the left hand side",
+      Iterator.fill(Depth)(NonZero).reduceLeft(_ union _))
+    testStackSafety("union is stack safe on the right hand side",
+      Iterator.fill(Depth)(NonZero).reduceRight(_ union _))
+    testStackSafety("intersection is stack safe on the left hand side",
+      Iterator.fill(Depth)(!NonZero).reduceLeft(_ intersection _))
+    testStackSafety("intersection is stack safe on the right hand side",
+      Iterator.fill(Depth)(!NonZero).reduceRight(_ intersection _))
+    testStackSafety("negation is stack safe",
+      Iterator.iterate(NonZero)(_.negate).drop(Depth).next())
+    testStackSafety("contramap() is stack safe",
+      Iterator.iterate(NonZero)(_.contramap(identity _)).drop(Depth).next())
+    testStackSafety("diff is stack safe on the left hand side",
+      Iterator.fill(Depth)(!NonZero).reduceLeft(_ diff _))
+    testStackSafety("diff is stack safe on the right hand side",
+      Iterator.fill(Depth)(!NonZero).reduceRight(_ diff _))
+  }
 }