Remove left-distributivity law from MonadCombine.

Also remove various obsolete/commented code.

js/src/test/scala/cats/tests/FutureTests.scala

@@ -51,31 +51,6 @@ class FutureTests extends CatsSuite {
   implicit def cogenForFuture[A]: Cogen[Future[A]] =
     Cogen[Unit].contramap(_ => ())
 
-  // FIXME: for some reason we aren't able to link against the correct
-  // function generator (Arbitrary.arbFunction1) here. i dont'
-  // understand why.  this method overrides the default implicit in
-  // scope.
-  //
-  // if you comment this method you'll get a compilation error:
-  //
-  //   Referring to non-existent method org.scalacheck.Arbitrary$.arbFunction1(org.scalacheck.Arbitrary)org.scalacheck.Arbitrary
-  //     called from cats.js.tests.FutureTests.<init>()
-  //     called from cats.js.tests.FutureTests.__exportedInits
-  //     exported to JavaScript with @JSExport
-  //   involving instantiated classes:
-  //     cats.js.tests.FutureTests
-  //
-  // if you look, you'll see that this is the wrong version of
-  // arbFunction1 -- it only takes an Arbitrary instead of an
-  // Arbitrary and a Cogen.
-  //
-  // what's going on? is this compiling against an earlier version of
-  // ScalaCheck? is something else happening? who knows?
-  //
-  // for now we'll let sleeping dogs lie.
-  //implicit def fakeArbitraryFunction[A: Cogen, B: Arbitrary]: Arbitrary[A => B] =
-  //  Arbitrary(arbitrary[B].map(b => (a: A) => b))
-
   checkAll("Future[Int]", MonadErrorTests[Future, Throwable].monadError[Int, Int, Int])
   checkAll("Future[Int]", ComonadTests[Future].comonad[Int, Int, Int])
   checkAll("Future", MonadTests[Future].monad[Int, Int, Int])

laws/src/main/scala/cats/laws/MonadCombineLaws.scala

@@ -9,8 +9,16 @@ import cats.syntax.all._
 trait MonadCombineLaws[F[_]] extends MonadFilterLaws[F] with AlternativeLaws[F] {
   implicit override def F: MonadCombine[F]
 
-  def monadCombineLeftDistributivity[A, B](fa: F[A], fa2: F[A], f: A => F[B]): IsEq[F[B]] =
-    F.combineK(fa, fa2).flatMap(f) <-> F.combineK(fa flatMap f, fa2 flatMap f)
+  // the left distributivity law does not hold for things like
+  // MonadCombine[Option]; here's a counter-example:
+  //
+  //     def f(x: Int): Option[Int] = if (x == 0) None else Some(x)
+  //     val a = Option(0)
+  //     val b = Option(1)
+  //     (a <+> b).flatMap(f) != (a.flatMap(f) <+> b.flatMap(f))
+  //
+  // def monadCombineLeftDistributivity[A, B](fa: F[A], fa2: F[A], f: A => F[B]): IsEq[F[B]] =
+  //   F.combineK(fa, fa2).flatMap(f) <-> F.combineK(fa flatMap f, fa2 flatMap f)
 }
 
 object MonadCombineLaws {

laws/src/main/scala/cats/laws/discipline/Arbitrary.scala

@@ -28,18 +28,6 @@ object arbitrary extends ArbitraryInstances0 {
   implicit def catsLawsCogenForFunction0[A](implicit A: Cogen[A]): Cogen[Function0[A]] =
     A.contramap(_())
 
-  // // A special function1Arbitrary for testing operations like dropWhile specifically
-  // // in the context of Int => Boolean. Once scalacheck supports better Function1 arbitrary
-  // // instances this can be removed.
-  // implicit def catsLawsArbitraryForIntToBool: Arbitrary[(Int) => Boolean] =
-  //   Arbitrary(
-  //     getArbitrary[Int].map(x =>
-  //      new Function1[Int, Boolean] {
-  //        def apply(i: Int): Boolean = i < x
-  //
-  //        override def toString = s"<function testing whether input is less than $x>"
-  //      }))
-
   implicit def catsLawsArbitraryForConst[A, B](implicit A: Arbitrary[A]): Arbitrary[Const[A, B]] =
     Arbitrary(A.arbitrary.map(Const[A, B]))
 

laws/src/main/scala/cats/laws/discipline/MonadCombineTests.scala

@@ -28,9 +28,10 @@ trait MonadCombineTests[F[_]] extends MonadFilterTests[F] with AlternativeTests[
       def name: String = "monadCombine"
       def bases: Seq[(String, RuleSet)] = Nil
       def parents: Seq[RuleSet] = Seq(monadFilter[A, B, C], alternative[A, B, C])
-      def props: Seq[(String, Prop)] = Seq(
-        "monadCombine left distributivity" -> forAll(laws.monadCombineLeftDistributivity[A, B] _)
-      )
+      def props: Seq[(String, Prop)] = Nil
+      // def props: Seq[(String, Prop)] = Seq(
+      //   "monadCombine left distributivity" -> forAll(laws.monadCombineLeftDistributivity[A, B] _)
+      // )
     }
   }
 }

tests/src/test/scala/cats/tests/CatsSuite.scala

@@ -11,11 +11,6 @@ import org.scalatest.{FunSuite, FunSuiteLike, Matchers}
 import org.scalatest.prop.{Configuration, GeneratorDrivenPropertyChecks}
 import org.typelevel.discipline.scalatest.Discipline
 
-//import org.scalacheck.{Arbitrary, Gen}
-//import org.scalacheck.Arbitrary.arbitrary
-
-//import scala.util.{Failure, Success, Try}
-
 trait TestSettings extends Configuration with Matchers {
 
   lazy val checkConfiguration: PropertyCheckConfiguration =
@@ -42,7 +37,6 @@ trait CatsSuite extends FunSuite
     with TestSettings
     with AllInstances
     with AllSyntax
-    with TestInstances
     with StrictCatsEquality { self: FunSuiteLike =>
 
   implicit override val generatorDrivenConfig: PropertyCheckConfiguration =
@@ -61,11 +55,3 @@ trait SlowCatsSuite extends CatsSuite {
   implicit override val generatorDrivenConfig: PropertyCheckConfiguration =
     slowCheckConfiguration
 }
-
-sealed trait TestInstances {
-  // // To be replaced by https://github.com/rickynils/scalacheck/pull/170
-  // implicit def arbitraryTry[A: Arbitrary]: Arbitrary[Try[A]] =
-  //   Arbitrary(Gen.oneOf(
-  //     arbitrary[A].map(Success(_)),
-  //     arbitrary[Throwable].map(Failure(_))))
-}

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

@@ -11,8 +11,7 @@ class OptionTests extends CatsSuite {
   checkAll("Option[Int]", CoflatMapTests[Option].coflatMap[Int, Int, Int])
   checkAll("CoflatMap[Option]", SerializableTests.serializable(CoflatMap[Option]))
 
-  // not actually lawful, fails distributivity
-  //checkAll("Option[Int]", MonadCombineTests[Option].monadCombine[Int, Int, Int])
+  checkAll("Option[Int]", MonadCombineTests[Option].monadCombine[Int, Int, Int])
   checkAll("MonadCombine[Option]", SerializableTests.serializable(MonadCombine[Option]))
 
   checkAll("Option[Int]", MonadTests[Option].monad[Int, Int, Int])