Add toNel syntax to List

core/src/main/scala/cats/data/package.scala

@@ -26,6 +26,12 @@ package object data {
       F.reduceRightTo(fa)(a => NonEmptyList(a, Nil)) { (a, lnel) =>
         lnel.map { case OneAnd(h, t) => OneAnd(a, h :: t) }
       }
+
+    def fromList[A](la: List[A]): Option[NonEmptyList[A]] =
+      la match {
+        case (h :: t) => Some(OneAnd(h, t))
+        case Nil => None
+      }
   }
 
   type ReaderT[F[_], A, B] = Kleisli[F, A, B]

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

@@ -15,6 +15,7 @@ trait AllSyntax
     with FunctorSyntax
     with GroupSyntax
     with InvariantSyntax
+    with ListSyntax
     with MonadCombineSyntax
     with MonadFilterSyntax
     with OptionSyntax

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

@@ -0,0 +1,12 @@
+package cats
+package syntax
+
+import cats.data.NonEmptyList
+
+trait ListSyntax {
+  implicit def listSyntax[A](la: List[A]): ListOps[A] = new ListOps(la)
+}
+
+final class ListOps[A](val la: List[A]) extends AnyVal {
+  def toNel: Option[NonEmptyList[A]] = NonEmptyList.fromList(la)
+}

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

@@ -14,8 +14,8 @@ object arbitrary {
   implicit def constArbitrary[A, B](implicit A: Arbitrary[A]): Arbitrary[Const[A, B]] =
     Arbitrary(A.arbitrary.map(Const[A, B]))
 
-  implicit def oneAndArbitrary[F[_], A](implicit A: Arbitrary[A], F: ArbitraryK[F]): Arbitrary[OneAnd[A, F]] =
-    Arbitrary(F.synthesize[A].arbitrary.flatMap(fa => A.arbitrary.map(a => OneAnd(a, fa))))
+  implicit def oneAndArbitrary[F[_], A](implicit A: Arbitrary[A], F: Arbitrary[F[A]]): Arbitrary[OneAnd[A, F]] =
+    Arbitrary(F.arbitrary.flatMap(fa => A.arbitrary.map(a => OneAnd(a, fa))))
 
   implicit def xorArbitrary[A, B](implicit A: Arbitrary[A], B: Arbitrary[B]): Arbitrary[A Xor B] =
     Arbitrary(Gen.oneOf(A.arbitrary.map(Xor.left), B.arbitrary.map(Xor.right)))

tests/shared/src/test/scala/cats/tests/ListTests.scala

@@ -1,9 +1,12 @@
 package cats
 package tests
 
+import cats.data.NonEmptyList
 import cats.laws.discipline.{TraverseTests, CoflatMapTests, MonadCombineTests, SerializableTests}
+import cats.laws.discipline.arbitrary._
+import org.scalatest.prop.GeneratorDrivenPropertyChecks
 
-class ListTests extends CatsSuite {
+class ListTests extends CatsSuite with GeneratorDrivenPropertyChecks {
   checkAll("List[Int]", CoflatMapTests[List].coflatMap[Int, Int, Int])
   checkAll("CoflatMap[List]", SerializableTests.serializable(CoflatMap[List]))
 
@@ -12,4 +15,14 @@ class ListTests extends CatsSuite {
 
   checkAll("List[Int] with Option", TraverseTests[List].traverse[Int, Int, Int, Int, Option, Option])
   checkAll("Traverse[List]", SerializableTests.serializable(Traverse[List]))
+
+  test("nel => list => nel returns original nel")(
+    forAll { fa: NonEmptyList[Int] =>
+      assert(fa.unwrap.toNel == Some(fa))
+    }
+  )
+
+  test("toNel on empty list returns None"){
+    assert(List.empty[Int].toNel == None)
+  }
 }