Add NonEmptyList.ofInitLast (#1910)

typelevel · Sep 13, 2017 · ca0204c · ca0204c
1 parent eff36ef
commit ca0204c
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diff --git a/core/src/main/scala/cats/data/NonEmptyList.scala b/core/src/main/scala/cats/data/NonEmptyList.scala
@@ -348,6 +348,12 @@ final case class NonEmptyList[+A](head: A, tail: List[A]) {
 object NonEmptyList extends NonEmptyListInstances {
   def of[A](head: A, tail: A*): NonEmptyList[A] = NonEmptyList(head, tail.toList)
 
+  def ofInitLast[A](init: List[A], last: A): NonEmptyList[A] =
+    init match {
+      case Nil => NonEmptyList(last, Nil)
+      case h :: t => NonEmptyList(h, t :+ last)
+    }
+
   def one[A](head: A): NonEmptyList[A] = NonEmptyList(head, Nil)
 
   /**

diff --git a/tests/src/test/scala/cats/tests/NonEmptyListTests.scala b/tests/src/test/scala/cats/tests/NonEmptyListTests.scala
@@ -74,6 +74,13 @@ class NonEmptyListTests extends CatsSuite {
     }
   }
 
+  test("Creating NonEmptyList with init/last + toList is identity") {
+    forAll { (init: List[Int], last: Int) =>
+      val list = init :+ last
+      val nonEmptyList = NonEmptyList.ofInitLast(init, last)
+      list should === (nonEmptyList.toList)
+    }
+  }
 
   test("NonEmptyList#filter is consistent with List#filter") {
     forAll { (nel: NonEmptyList[Int], p: Int => Boolean) =>