From d571771ef5c8b692b9b015263e3b3d0f26c49298 Mon Sep 17 00:00:00 2001
From: Luka Jacobowitz <luka.jacobowitz@codecentric.de>
Date: Wed, 30 Aug 2017 17:31:25 +0200
Subject: [PATCH] Add sequential roundtrip law

---
 laws/src/main/scala/cats/laws/ParallelLaws.scala     |  3 +++
 .../scala/cats/laws/discipline/ParallelTests.scala   | 12 +++++++-----
 2 files changed, 10 insertions(+), 5 deletions(-)

diff --git a/laws/src/main/scala/cats/laws/ParallelLaws.scala b/laws/src/main/scala/cats/laws/ParallelLaws.scala
index ad79ac05e68..dfaa894d01c 100644
--- a/laws/src/main/scala/cats/laws/ParallelLaws.scala
+++ b/laws/src/main/scala/cats/laws/ParallelLaws.scala
@@ -11,6 +11,9 @@ trait ParallelLaws[M[_], F[_]] {
 
   def parallelRoundTrip[A](ma: M[A]): IsEq[M[A]] =
     P.sequential(monadM)(P.parallel(monadM)(ma)) <-> ma
+
+  def sequentialRoundTrip[A](fa: F[A]): IsEq[F[A]] =
+    P.parallel(monadM)(P.sequential(monadM)(fa)) <-> fa
 }
 
 object ParallelLaws {
diff --git a/laws/src/main/scala/cats/laws/discipline/ParallelTests.scala b/laws/src/main/scala/cats/laws/discipline/ParallelTests.scala
index 8fc2af69305..0d269597ffb 100644
--- a/laws/src/main/scala/cats/laws/discipline/ParallelTests.scala
+++ b/laws/src/main/scala/cats/laws/discipline/ParallelTests.scala
@@ -9,11 +9,13 @@ import org.typelevel.discipline.Laws
 trait ParallelTests[M[_], F[_], A] extends Laws {
   def laws: ParallelLaws[M, F]
 
-  def parallel(implicit ArbM: Arbitrary[M[A]], EqMa: Eq[M[A]]): RuleSet = new DefaultRuleSet(
-    "parallel",
-    None,
-    "parallel round trip" -> forAll((ma: M[A]) => laws.parallelRoundTrip(ma))
-  )
+  def parallel(implicit ArbM: Arbitrary[M[A]], EqMa: Eq[M[A]], ArbF: Arbitrary[F[A]], EqFa: Eq[F[A]]): RuleSet =
+    new DefaultRuleSet(
+      "parallel",
+      None,
+      "parallel round trip" -> forAll((ma: M[A]) => laws.parallelRoundTrip(ma)),
+      "sequential round trip" -> forAll((fa: F[A]) => laws.sequentialRoundTrip(fa))
+    )
 }
 
 object ParallelTests {