Comment by movpasd

I think it can be useful to think of the parsing and logic parts both as modules, with the parsing part interfacing with the outside world via unstructured data, and the parsing and logic parts interfacing via structured data, i.e.: the validated types.

From that perspective, there is a clear trade-off on the size of the parsing–logic interface. Introducing more granular, safer validated types may give you better functionality, but it forces you to expand that interface and create coupling.

I think there is a middle ground, which is that these safe types should be chunked into larger structures that enforce a range of related invariants and hopefully have some kind of domain meaning. That way, you shrink the conceptual surface area of the interface so that working with it is less painful.