Scala-519: flattening (#512)

* Create unit tests for a full flattener

* Write a "library pimper" for full flattening

* Inline the pointless value flattenedList

* Add a test that mixes several types of collection

* Rename fullFlat to fullFlatten

Author: bruno-unna

scala-core-collections/src/main/scala/com/baeldung/scala/flattening/Flattener.scala

@@ -0,0 +1,23 @@
+package com.baeldung.scala.flattening
+
+import scala.language.implicitConversions
+
+object Flattener {
+
+  /** This wrapper "pimps" the Seq type, adding the `fullFlat` method.
+    *
+    * @param seq
+    *   sequence whose functionality will be expanded
+    * @return
+    *   a wrapper object that implements the `fullFlat` method
+    */
+  implicit def sequenceFlattener(seq: Seq[Any]): FullFlat =
+    new FullFlat(seq)
+
+  class FullFlat(seq: Seq[Any]) {
+    def fullFlatten: Seq[Any] = seq flatten {
+      case seq: Seq[Any] => seq.fullFlatten
+      case nonSeq        => Seq(nonSeq)
+    }
+  }
+}

scala-core-collections/src/test/scala/com/baedung/scala/flattening/FlattenerSpec.scala

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+package com.baedung.scala.flattening
+
+import org.scalatest.wordspec.AnyWordSpec
+
+import scala.collection.immutable.Queue
+
+class FlattenerSpec extends AnyWordSpec {
+
+  import com.baeldung.scala.flattening.Flattener.sequenceFlattener
+
+  "A full flattener" should {
+    "respect the contents of an already flat sequence" in {
+      val flatList = List(3, 7, 2, 7, 1, 3, 4)
+      assertResult(flatList)(flatList.fullFlatten)
+    }
+    "flatten a nested empty list to an empty list" in {
+      val list = List(List(List()))
+      assertResult(List.empty)(list.fullFlatten)
+    }
+    "flatten several lists of the same type, one level deep" in {
+      val list = List(
+        List(1, 2, 3),
+        List(4, 5),
+        List(6)
+      )
+      assertResult(List(1, 2, 3, 4, 5, 6))(list.fullFlatten)
+    }
+    "flatten several lists of the same type, diverse levels deep" in {
+      val list = List(
+        List(1, List(2, 3)),
+        List(4, List(List(5))),
+        List(6)
+      )
+      assertResult(List(1, 2, 3, 4, 5, 6))(list.fullFlatten)
+    }
+    "flatten diverse types of collections" in {
+      val list = List(
+        Vector(1, Queue(2, 3))
+      )
+      assertResult(List(1, 2, 3))(list.fullFlatten)
+    }
+    "flatten several lists of the diverse types, diverse levels deep" in {
+      val list = List(
+        List(1, List("b", 'c')),
+        List(4.4, List(List(5)))
+      )
+      assertResult(List(1, "b", 'c', 4.4, 5))(list.fullFlatten)
+    }
+  }
+}