add init and size methods to NonEmptyList (#1628)

* add init and size methods to NonEmptyList

* add tests

* change NonEmptyList.last implementation

* Revert "change NonEmptyList.last implementation"

This reverts commit 5745f5b.

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

@@ -17,14 +17,56 @@ final case class NonEmptyList[+A](head: A, tail: List[A]) {
 
   /**
    * Return the head and tail into a single list
+   * {{{
+   * scala> import cats.data.NonEmptyList
+   * scala> val nel = NonEmptyList.of(1, 2, 3, 4, 5)
+   * scala> nel.toList
+   * res0: scala.collection.immutable.List[Int] = List(1, 2, 3, 4, 5)
+   * }}}
    */
   def toList: List[A] = head :: tail
 
+  /**
+   * Selects the last element
+   * {{{
+   * scala> import cats.data.NonEmptyList
+   * scala> val nel = NonEmptyList.of(1, 2, 3, 4, 5)
+   * scala> nel.last
+   * res0: Int = 5
+   * }}}
+   */
   def last: A = tail.lastOption match {
     case None    => head
     case Some(a) => a
   }
 
+  /**
+   * Selects all elements except the last
+   *
+   * {{{
+   * scala> import cats.data.NonEmptyList
+   * scala> val nel = NonEmptyList.of(1, 2, 3, 4, 5)
+   * scala> nel.init
+   * res0: scala.collection.immutable.List[Int] = List(1, 2, 3, 4)
+   * }}}
+   */
+  def init: List[A] = tail match {
+    case Nil => List.empty
+    case t => head :: t.init
+  }
+
+  /**
+   * The size of this NonEmptyList
+   *
+   * {{{
+   * scala> import cats.data.NonEmptyList
+   * scala> val nel = NonEmptyList.of(1, 2, 3, 4, 5)
+   * scala> nel.size
+   * res0: Int = 5
+   * }}}
+   */
+  def size: Int = 1 + tail.size
+
   /**
    *  Applies f to all the elements of the structure
    */

tests/src/test/scala/cats/tests/NonEmptyListTests.scala

@@ -238,6 +238,18 @@ class NonEmptyListTests extends CatsSuite {
     }
   }
 
+  test("NonEmptyList#init is consistent with List#init") {
+    forAll { nel: NonEmptyList[Int] =>
+      nel.init should === (nel.toList.init)
+    }
+  }
+
+  test("NonEmptyList#size is consistent with List#size") {
+    forAll { nel: NonEmptyList[Int] =>
+      nel.size should === (nel.toList.size)
+    }
+  }
+
   test("NonEmptyList#sorted is consistent with List#sorted") {
     forAll { nel: NonEmptyList[Int] =>
       nel.sorted.toList should === (nel.toList.sorted)