I don't know what you mean by your first example, how does that lead to using undefined data? And your second example is incorrect, if you use an Either in mon

I don't know what you mean by your first example, how does that lead to using undefined data? And your second example is incorrect, if you use an Either in monadic style then each successive function call after the error occurs is doing nothing. It simply returns the existing error from the Either, it doesn't try to run a computation on the non-existent value.