Merge pull request #1130 from hamishdickson/add-monoid-syntax

Add monoid syntax

core/src/main/scala/cats/syntax/all.scala

@@ -24,6 +24,7 @@ trait AllSyntax
     with MonadCombineSyntax
     with MonadErrorSyntax
     with MonadFilterSyntax
+    with MonoidSyntax
     with OptionSyntax
     with OrderSyntax
     with PartialOrderSyntax

core/src/main/scala/cats/syntax/monoid.scala

@@ -0,0 +1,12 @@
+package cats
+package syntax
+
+trait MonoidSyntax extends SemigroupSyntax {
+  // TODO: use simulacrum instances eventually
+  implicit def catsSyntaxMonoid[A: Monoid](a: A): MonoidOps[A] =
+    new MonoidOps[A](a)
+}
+
+final class MonoidOps[A: Monoid](lhs: A) {
+  def isEmpty(implicit eq: Eq[A]): Boolean = Monoid[A].isEmpty(lhs)(eq)
+}

tests/src/test/scala/cats/tests/SyntaxTests.scala

@@ -40,6 +40,12 @@ class SyntaxTests extends AllInstances with AllSyntax {
     val z: A = x |-| y
   }
 
+  def testMonoid[A: Monoid]: Unit = {
+    val x = mock[A]
+    implicit val y = mock[Eq[A]]
+    val z: Boolean = x.isEmpty
+  }
+
   def testEq[A: Eq]: Unit = {
     val x = mock[A]
     val y = mock[A]