rearrange builtins

builtins/typeof.go

@@ -0,0 +1,12 @@
+package builtins
+
+import "monkey/object"
+
+// TypeOf  ...
+func TypeOf(args ...object.Object) object.Object {
+	if len(args) != 1 {
+		return newError("wrong number of arguments. got=%d, want=1", len(args))
+	}
+
+	return &object.String{Value: string(args[0].Type())}
+}