Null my old friend

vm/vm.go

@@ -9,6 +9,7 @@ import (
 
 const StackSize = 2048
 
+var Null = &object.Null{}
 var True = &object.Boolean{Value: true}
 var False = &object.Boolean{Value: false}
 
@@ -99,6 +100,12 @@ func (vm *VM) Run() error {
 			if !isTruthy(condition) {
 				ip = pos - 1
 			}
+
+		case code.OpNull:
+			err := vm.push(Null)
+			if err != nil {
+				return err
+			}
 		}
 	}
 
@@ -111,6 +118,9 @@ func isTruthy(obj object.Object) bool {
 	case *object.Boolean:
 		return obj.Value
 
+	case *object.Null:
+		return false
+
 	default:
 		return true
 	}
@@ -211,6 +221,8 @@ func (vm *VM) executeBangOperator() error {
 		return vm.push(False)
 	case False:
 		return vm.push(True)
+	case Null:
+		return vm.push(True)
 	default:
 		return vm.push(False)
 	}

vm/vm_test.go

@@ -54,6 +54,11 @@ func testExpectedObject(t *testing.T, expected interface{}, actual object.Object
 		if err != nil {
 			t.Errorf("testBooleanObject failed: %s", err)
 		}
+
+	case *object.Null:
+		if actual != Null {
+			t.Errorf("object is not Null: %T (%+v)", actual, actual)
+		}
 	}
 }
 
@@ -138,6 +143,7 @@ func TestBooleanExpressions(t *testing.T) {
 		{"!!true", true},
 		{"!!false", false},
 		{"!!5", true},
+		{"!(if (false) { 5; })", true},
 	}
 
 	runVmTests(t, tests)
@@ -152,6 +158,9 @@ func TestConditionals(t *testing.T) {
 		{"if (1 < 2) { 10 }", 10},
 		{"if (1 < 2) { 10 } else { 20 }", 10},
 		{"if (1 > 2) { 10 } else { 20 }", 20},
+		{"if (1 > 2) { 10 }", Null},
+		{"if (false) { 10 }", Null},
+		{"if ((if (false) { 10 })) { 10 } else { 20 }", 20},
 	}
 
 	runVmTests(t, tests)