You can’t catch undefined, and if you want to avoid using both that and other exceptions, you’re going to have some trouble with computations getting wrapped in increasingly complex layers of Maybe and Either in your pure code, where you may not necessarily want to handle them yet. Exceptions are a little bit more implicit (like undefined) but can still be handled by code as they propagate through the IO part of the program. In addition, instances of exception can carry information that can be quite helpful at times, like on what line and column an unexpected symbol was found.