Add null literal

evaluator/evaluator.go

@@ -87,6 +87,9 @@ func Eval(node ast.Node, env *object.Environment) object.Object {
 	case *ast.Boolean:
 		return nativeBoolToBooleanObject(node.Value)
 
+	case *ast.Null:
+		return NULL
+
 	case *ast.PrefixExpression:
 		right := Eval(node.Right, env)
 		if isError(right) {

evaluator/evaluator_test.go

@@ -623,6 +623,11 @@ func testIntegerObject(t *testing.T, obj object.Object, expected int64) bool {
 	return true
 }
 
+func TestNullExpression(t *testing.T) {
+	evaluated := testEval("null")
+	testNullObject(t, evaluated)
+}
+
 func testBooleanObject(t *testing.T, obj object.Object, expected bool) bool {
 	result, ok := obj.(*object.Boolean)
 	if !ok {