added assignment 7 ex 1

src/Assignment7/Ex1.sc

@@ -0,0 +1,35 @@
+trait Stack[+A] {
+    def push[B >: A](elem: B) : Stack[B] = ElemStack(elem, this)
+    def top: A
+    def pop: Stack[A]
+}
+
+case class EmptyStack[+A]() extends Stack[A] {
+  override def top: A = throw new IndexOutOfBoundsException("Stack is empty")
+  override def pop: Stack[A] = this
+}
+
+case class ElemStack[+A](elmt: A, base: Stack[A]) extends Stack[A] {
+  override def top: A = elmt
+  override def pop: Stack[A] = base
+  override def toString: String = elmt.toString + "," + base.toString
+}
+
+// Construction, pop and toString
+val a = EmptyStack().push("hello").push("world").push("it's fun").pop
+assert(a.toString() == "world,hello,EmptyStack()")
+
+// Getting top
+val b = EmptyStack().push(1).push(3)
+assert(b.top == 3)
+
+// Variance checks
+class Foo
+class Bar extends Foo
+val c: Stack[Bar] = EmptyStack().push(new Bar()).push(new Bar())
+assert(c.top.isInstanceOf[Bar] == true)
+assert(c.top.isInstanceOf[Foo] == true)
+
+// Variance check 2
+val d: Stack[Foo] = EmptyStack().push(new Bar()).push(new Bar())
+assert(d.top.isInstanceOf[Foo])