Okay but the solution here is to identify and parcel your cases into discrete entities. The article doesn't say "don't accept anything odd", it says "clearly identify what you accept". If you have to accept odd cases, identify them so it's clear what's happening.

So this isn't about purity, it's about being declarative. i.e. make your code say what it accepts, instead of writing board/implicit acceptable inputs that inevitably forget cases and crashes.

If you limit what you accept as inputs then you can stop worrying about downstream error handling and debugging.