Test associativity of (Co)Kleisli composition

closes typelevel#732

tests/src/test/scala/cats/tests/OptionTests.scala

@@ -1,6 +1,7 @@
 package cats
 package tests
 
+import cats.laws.{CoflatMapLaws, FlatMapLaws}
 import cats.laws.discipline.{TraverseTests, CoflatMapTests, MonadCombineTests, SerializableTests}
 
 class OptionTests extends CatsSuite {
@@ -21,4 +22,30 @@ class OptionTests extends CatsSuite {
       fs.show should === (fs.toString)
     }
   }
+
+  // The following two tests check the kleisliAssociativity and
+  // cokleisliAssociativity laws which are a different formulation of
+  // the flatMapAssociativity and coflatMapAssociativity laws. Since
+  // these laws are more or less duplicates of existing laws, we don't
+  // check them for all types that have FlatMap or CoflatMap instances.
+
+  test("Kleisli associativity") {
+    forAll { (l: Long,
+              f: Long => Option[Int],
+              g: Int  => Option[Char],
+              h: Char => Option[String]) =>
+      val isEq = FlatMapLaws[Option].kleisliAssociativity(f, g, h, l)
+      isEq.lhs should === (isEq.rhs)
+    }
+  }
+
+  test("Cokleisli associativity") {
+    forAll { (l: Option[Long],
+              f: Option[Long] => Int,
+              g: Option[Int]  => Char,
+              h: Option[Char] => String) =>
+      val isEq = CoflatMapLaws[Option].cokleisliAssociativity(f, g, h, l)
+      isEq.lhs should === (isEq.rhs)
+    }
+  }
 }