Add Semigroupal instance for Monoid

typelevel · Oct 17, 2017 · 64cd30d · 64cd30d
1 parent 60aa276
commit 64cd30d
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Showing 4 changed files with 12 additions and 5 deletions.
diff --git a/core/src/main/scala/cats/Semigroupal.scala b/core/src/main/scala/cats/Semigroupal.scala
@@ -16,4 +16,11 @@ import simulacrum.typeclass
   def product[A, B](fa: F[A], fb: F[B]): F[(A, B)]
 }
 
-object Semigroupal extends SemigroupalArityFunctions
+object Semigroupal extends SemigroupalArityFunctions {
+  implicit def catsSemigroupalForMonoid: Semigroupal[Monoid] = new Semigroupal[Monoid] {
+    def product[A, B](fa: Monoid[A], fb: Monoid[B]): Monoid[(A, B)] = new Monoid[(A, B)] {
+      val empty = fa.empty -> fb.empty
+      def combine(x: (A, B), y: (A, B)): (A, B) = fa.combine(x._1, y._1) -> fb.combine(x._2, y._2)
+    }
+  }
+}
diff --git a/core/src/main/scala/cats/instances/invariant.scala b/core/src/main/scala/cats/instances/invariant.scala
@@ -45,7 +45,4 @@ trait InvariantMonoidalInstances {
     }
   }
 
-
-
-
 }
diff --git a/tests/src/test/scala/cats/tests/AlgebraInvariantTests.scala b/tests/src/test/scala/cats/tests/AlgebraInvariantTests.scala
@@ -4,7 +4,7 @@ package tests
 import cats.Invariant
 import cats.kernel._
 import cats.kernel.laws.discipline.{SemigroupTests => SemigroupLawTests, MonoidTests => MonoidLawTests, GroupTests => GroupLawTests, _}
-import cats.laws.discipline.{InvariantMonoidalTests, InvariantTests, SerializableTests}
+import cats.laws.discipline.{InvariantMonoidalTests, InvariantTests, SerializableTests, SemigroupalTests}
 import cats.laws.discipline.eq._
 import org.scalacheck.{Arbitrary, Gen}
 
@@ -64,6 +64,7 @@ class AlgebraInvariantTests extends CatsSuite {
   checkAll("InvariantMonoidal[Semigroup]", SemigroupLawTests[Int](InvariantMonoidal[Semigroup].pure(0)).semigroup)
   checkAll("InvariantMonoidal[CommutativeSemigroup]", CommutativeSemigroupTests[Int](InvariantMonoidal[CommutativeSemigroup].pure(0)).commutativeSemigroup)
 
+  checkAll("Semigroupal[Monoid]", SemigroupalTests[Monoid].semigroupal[Int, Int, Int])
 
 
   {

diff --git a/tests/src/test/scala/cats/tests/MonoidTests.scala b/tests/src/test/scala/cats/tests/MonoidTests.scala
@@ -6,6 +6,7 @@ package tests
 class MonoidTests extends CatsSuite {
   {
     Invariant[Monoid]
+    Semigroupal[Monoid]
   }
 
   test("companion object syntax") {
@@ -18,6 +19,7 @@ class MonoidTests extends CatsSuite {
 object MonoidTests {
   def summonInstance(): Unit = {
     Invariant[Monoid]
+    Semigroupal[Monoid]
     ()
   }