[cadamsdotcom]

Parsing over validation, and newtypes for everything, fall over when you don’t know the full range of possibilities that can occur in the wild.

It is a handy way to prevent divide by zero as in the article, or to have fun with lambda calculus by asking the type system if 3 + 4 == 8. You can reason about the full range of inputs. Same for file format parsing - making as many failure modes as possible fail as early as possible!

But be VERY wary of using them to represent business logic or state machines that allow only the transitions you believe can exist at this point in time. You just don’t know what wild things people will want to do in business logic, and if your software can’t handle those scenarios, people will just work around it and your stuff no longer matches reality.