As I said, it will get super-ugly, and it has not been done (in any language with more than 1 user), I think. Why? Because you will want an error class for a whole tree of classes you define, and it is not so trivially clear how that should look like. A simple 'bottom' (i.e., 'null') works. But e.g. you have 'Expr' for your expressions and you want 'ExprError' to be your error class for that that subclasses all 'Expr' and is a superclass of bottom. Now when you define 'ExprPlus' and 'ExprMinus' and 'ExprInt' and so on, all subclasses of 'Expr', you still want 'ExprError' to be a subclass of those to indicate an error. That is the difficult part: how to express exactly what you want? How does the inheritance graph look like? At that point, languages introduced exceptions. And after that: generic error classes: 'Optional<Expr>' and 'Error<Expr>', etc., without a global 'bottom'. This forces you to think about an error case: you cannot just return ExprError from anything that returns Expr, but you need to tell the compiler that you will return 'Optional<Expr>' so the caller is forced to handle this somehow.