Add DistributiveLattice instance for HashSet

typelevel · Aug 2, 2022 · aaa8a0a · aaa8a0a
1 parent 462709e
commit aaa8a0a
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Showing 2 changed files with 41 additions and 11 deletions.
diff --git a/core/src/main/scala/cats/collections/HashSet.scala b/core/src/main/scala/cats/collections/HashSet.scala
@@ -41,6 +41,7 @@
 
 package cats.collections
 
+import algebra.lattice.DistributiveLattice
 import cats.Foldable
 import cats.Show
 import cats.UnorderedFoldable
@@ -1108,7 +1109,7 @@ object HashSet extends compat.HashSetCompatCompanion {
     }
   }
 
-  implicit val catsDataUnorderedFoldableForHashSet: UnorderedFoldable[HashSet] =
+  implicit val catsCollectionsUnorderedFoldableForHashSet: UnorderedFoldable[HashSet] =
     new UnorderedFoldable[HashSet] {
       override def isEmpty[A](fa: HashSet[A]): Boolean = fa.isEmpty
 
@@ -1127,18 +1128,37 @@ object HashSet extends compat.HashSetCompatCompanion {
         C.combineAll(fa.iterator.map(f))
     }
 
-  implicit def catsDataCommutativeMonoidForHashSet[A](implicit hash: Hash[A]): CommutativeMonoid[HashSet[A]] =
-    new CommutativeMonoid[HashSet[A]] {
-      def empty: HashSet[A] = HashSet.empty[A]
-      def combine(x: HashSet[A], y: HashSet[A]): HashSet[A] = x.union(y)
-    }
+  implicit def catsCollectionsCommutativeMonoidForHashSet[A](implicit hash: Hash[A]): CommutativeMonoid[HashSet[A]] =
+    new HashSetUnionMonoid[A]
 
-  implicit def catsDataShowForHashSet[A](implicit A: Show[A]): Show[HashSet[A]] =
+  implicit def catsCollectionsShowForHashSet[A](implicit A: Show[A]): Show[HashSet[A]] =
     Show.show[HashSet[A]](_.show)
 
-  implicit def catsDataHashForHashSet[A](implicit A: Hash[A]): Hash[HashSet[A]] =
+  implicit def catsCollectionsHashForHashSet[A](implicit A: Hash[A]): Hash[HashSet[A]] =
     new Hash[HashSet[A]] {
       def hash(hs: HashSet[A]): Int = hs.hashCode
       def eqv(x: HashSet[A], y: HashSet[A]): Boolean = x === y
     }
+
+  implicit def catsCollectionsDistributiveLatticeForHashSet[A](implicit A: Hash[A]): DistributiveLattice[HashSet[A]] =
+    new DistributiveLattice[HashSet[A]] {
+      private val joinMonoid = new HashSetUnionMonoid[A]
+      private val meetMonoid = new HashSetIntersectionMonoid[A]
+
+      override def join(lhs: HashSet[A], rhs: HashSet[A]): HashSet[A] =
+        joinMonoid.combine(lhs, rhs)
+
+      override def meet(lhs: HashSet[A], rhs: HashSet[A]): HashSet[A] =
+        meetMonoid.combine(lhs, rhs)
+    }
+}
+
+class HashSetUnionMonoid[A: Hash] extends CommutativeMonoid[HashSet[A]] {
+  override def empty: HashSet[A] = HashSet.empty[A]
+  override def combine(x: HashSet[A], y: HashSet[A]): HashSet[A] = x.union(y)
+}
+
+class HashSetIntersectionMonoid[A: Hash] extends CommutativeMonoid[HashSet[A]] {
+  override def empty: HashSet[A] = HashSet.empty[A]
+  override def combine(x: HashSet[A], y: HashSet[A]): HashSet[A] = x.intersect(y)
 }
diff --git a/tests/src/test/scala/cats/collections/HashSetSuite.scala b/tests/src/test/scala/cats/collections/HashSetSuite.scala
@@ -21,6 +21,7 @@
 
 package cats.collections
 
+import algebra.laws.LatticeLaws
 import cats.collections.arbitrary.hashset._
 import cats.kernel.laws.discipline.CommutativeMonoidTests
 import cats.kernel.laws.discipline.HashTests
@@ -41,14 +42,23 @@ class HashSetSuite extends DisciplineSuite {
     DefaultScalaCheckPropertyCheckConfig.default
 
   checkAll("HashSet[Int]", HashTests[HashSet[Int]].hash)
-  checkAll("Hash[HashSet[Int]]", SerializableTests.serializable(HashSet.catsDataHashForHashSet[Int]))
+  checkAll("Hash[HashSet[Int]]", SerializableTests.serializable(HashSet.catsCollectionsHashForHashSet[Int]))
 
   checkAll("HashSet[Int]", UnorderedFoldableTests[HashSet].unorderedFoldable[Int, Int])
-  checkAll("UnorderedFoldable[HashSet]", SerializableTests.serializable(HashSet.catsDataUnorderedFoldableForHashSet))
+  checkAll("UnorderedFoldable[HashSet]",
+           SerializableTests.serializable(HashSet.catsCollectionsUnorderedFoldableForHashSet)
+  )
 
   checkAll("HashSet[String]", CommutativeMonoidTests[HashSet[String]].commutativeMonoid)
+
   checkAll("CommutativeMonoid[HashSet[String]]",
-           SerializableTests.serializable(HashSet.catsDataCommutativeMonoidForHashSet[String])
+           SerializableTests.serializable(HashSet.catsCollectionsCommutativeMonoidForHashSet[String])
+  )
+
+  checkAll("HashSet[String]", LatticeLaws[HashSet[String]].distributiveLattice)
+
+  checkAll("DistributiveLattice[HashSet[String]]",
+           SerializableTests.serializable(HashSet.catsCollectionsDistributiveLatticeForHashSet[String])
   )
 
   // Based on https://stackoverflow.com/questions/9406775/why-does-string-hashcode-in-java-have-many-conflicts