added lesson 5

src/Lesson5/Ex5_2.sc

@@ -0,0 +1,19 @@
+def length[A](x: List[A]): Int = {
+  x.foldRight(0)((elem: A, len: Int) => len + 1)
+}
+
+def map[A, B](x: List[A], f: A => B): List[B] = {
+  x.foldRight(List.empty[B])((elem: A, list: List[B]) => f(elem)::list)
+}
+
+def dup[A](l: List[A]): List[A] = {
+  l.flatMap(e => List(e, e))
+}
+
+def dup2[A](l: List[A]): List[A] = {
+  l.foldRight(List.empty[A])((e, l) => e::e::l)
+}
+
+length(1::2::3::4::Nil)
+map(1::2::3::4::Nil, (x: Int) => x * 2)
+dup(1::2::3::Nil)