[draven]

The validation is not completely thrown away, since the type indicates that the data has been validated. I understand "parsing" as applying more structure to a piece of data. Going from a String to an IP or a Username fits the definition.

I push my team to use this pattern in our (mostly Scala) codebase. We have too many instances of useless validations, because the fact that a piece of data has been "parsed"/validated is not reflected in its type using simple validation.

For example using String, a function might validate the String as a Username. Lower in the call stack, a function ends up taking this String as an arg. It has no way of knowing if it has been validated or not and has to re-validate it. If the first validation gets a Username as a result, other functions down the call stack can take a Username as an argument and know for sure it's been validated / "parsed".