Add id

typelevel · Sep 27, 2017 · 9026c96 · 9026c96
1 parent 53343b3
commit 9026c96
Show file tree

Hide file tree

Showing 2 changed files with 11 additions and 4 deletions.
diff --git a/kernel-laws/src/main/scala/cats/kernel/laws/MonoidLaws.scala b/kernel-laws/src/main/scala/cats/kernel/laws/MonoidLaws.scala
@@ -1,6 +1,6 @@
 package cats.kernel.laws
 
-import cats.kernel.Monoid
+import cats.kernel.{Eq, Monoid}
 
 trait MonoidLaws[A] extends SemigroupLaws[A] {
   override implicit def S: Monoid[A]
@@ -20,6 +20,10 @@ trait MonoidLaws[A] extends SemigroupLaws[A] {
   def combineAll(xs: Vector[A]): IsEq[A] =
     S.combineAll(xs) <-> (S.empty +: xs).reduce(S.combine)
 
+  def isId(x: A, eqv: Eq[A]): IsEq[Boolean] = {
+    eqv.eqv(x, S.empty) <-> S.isEmpty(x)(eqv)
+  }
+
 }
 
 object MonoidLaws {

diff --git a/kernel-laws/src/main/scala/cats/kernel/laws/discipline/MonoidLawTests.scala b/kernel-laws/src/main/scala/cats/kernel/laws/discipline/MonoidLawTests.scala
@@ -1,7 +1,9 @@
-package cats.kernel.laws.discipline
+package cats
+package kernel
+package laws
+package discipline
 
-import cats.kernel.{Monoid, Eq}
-import cats.kernel.laws.MonoidLaws
+import cats.kernel.instances.boolean._
 import org.scalacheck.Arbitrary
 import org.scalacheck.Prop.forAll
 
@@ -17,6 +19,7 @@ trait MonoidLawTests[A] extends SemigroupLawTests[A] {
       "right identity" -> forAll(laws.rightIdentity _),
       "combine all" -> forAll(laws.combineAll _),
       "collect0" -> forAll(laws.collect0 _),
+      "is id" -> forAll((a: A) => laws.isId(a, eqA)),
       "repeat0" -> forAll(laws.repeat0 _))
 
 }