Add map method to OneAnd. Fix for #885

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

@@ -3,6 +3,7 @@ package data
 
 import scala.annotation.tailrec
 import scala.collection.mutable.ListBuffer
+import cats.std.list._
 
 /**
  * A data type which represents a single element (head) and some other
@@ -65,6 +66,12 @@ final case class OneAnd[F[_], A](head: A, tail: F[A]) {
   def foldRight[B](lb: Eval[B])(f: (A, Eval[B]) => Eval[B])(implicit F: Foldable[F]): Eval[B] =
     Eval.defer(f(head, F.foldRight(tail, lb)(f)))
 
+  /**
+    * Applies f to all the elements of the structure
+    */
+  def map[B](f: A => B)(implicit F: Functor[F]): OneAnd[F, B] =
+    OneAnd(f(head), F.map(tail)(f))
+
   /**
    * Typesafe equality operator.
    *
@@ -104,7 +111,7 @@ private[data] sealed trait OneAndInstances extends OneAndLowPriority2 {
     }
 
   implicit def oneAndSemigroup[F[_]: MonadCombine, A]: Semigroup[OneAnd[F, A]] =
-    oneAndSemigroupK.algebra
+    oneAndSemigroupK(MonadCombine[F]).algebra
 
   implicit def oneAndReducible[F[_]](implicit F: Foldable[F]): Reducible[OneAnd[F, ?]] =
     new NonEmptyReducible[OneAnd[F,?], F] {
@@ -114,7 +121,7 @@ private[data] sealed trait OneAndInstances extends OneAndLowPriority2 {
   implicit def oneAndMonad[F[_]](implicit monad: MonadCombine[F]): Monad[OneAnd[F, ?]] =
     new Monad[OneAnd[F, ?]] {
       override def map[A, B](fa: OneAnd[F, A])(f: A => B): OneAnd[F, B] =
-        OneAnd(f(fa.head), monad.map(fa.tail)(f))
+        fa map f
 
       def pure[A](x: A): OneAnd[F, A] =
         OneAnd(x, monad.empty)
@@ -133,7 +140,6 @@ private[data] sealed trait OneAndInstances extends OneAndLowPriority2 {
 trait OneAndLowPriority0 {
   implicit val nelComonad: Comonad[OneAnd[List, ?]] =
     new Comonad[OneAnd[List, ?]] {
-
       def coflatMap[A, B](fa: OneAnd[List, A])(f: OneAnd[List, A] => B): OneAnd[List, B] = {
         @tailrec def consume(as: List[A], buf: ListBuffer[B]): List[B] =
           as match {
@@ -147,15 +153,15 @@ trait OneAndLowPriority0 {
         fa.head
 
       def map[A, B](fa: OneAnd[List, A])(f: A => B): OneAnd[List, B] =
-        OneAnd(f(fa.head), fa.tail.map(f))
+        fa map f
     }
 }
 
 trait OneAndLowPriority1 extends OneAndLowPriority0 {
   implicit def oneAndFunctor[F[_]](implicit F: Functor[F]): Functor[OneAnd[F, ?]] =
     new Functor[OneAnd[F, ?]] {
       def map[A, B](fa: OneAnd[F, A])(f: A => B): OneAnd[F, B] =
-        OneAnd(f(fa.head), F.map(fa.tail)(f))
+        fa map f
     }
 
 }

tests/src/test/scala/cats/tests/OneAndTests.scala

@@ -109,4 +109,11 @@ class OneAndTests extends CatsSuite {
       nel.forall(p) should === (list.forall(p))
     }
   }
+
+  test("NonEmptyList#map is consistent with List#map") {
+    forAll { (nel: NonEmptyList[Int], p: Int => String) =>
+      val list = nel.unwrap
+      nel.map(p).unwrap should === (list.map(p))
+    }
+  }
 }