Well, if you want to collect errors, then you need to have a way to store the transformed input in a form that allows you to check the invariants, which can be arbitrarily complex. So naturally there must be some intermediate representations that allow illegal states. And there must be functions that take these IRs that return either domain objects or lists of errors.

So, having used this thread to rubber-duck about how the principle of "parse-don't-validate" works with the principle of "provide good error messages", I'm arriving at these rules, which are really more about encapsulation than parsing:

1. Encapsulate both parsing and validation in a single function: `parse(RawInput) -> Result<ValidDomainObject,ListOfErrors>`

2. Ideally, `parse` is implemented by a robust parsing/validation library for the type of input that you're dealing with. It will create some intermediate representations that you need not concern yourself with.

3. If there isn't a good parser library for your use case, your implementation of `parse` will necessarily contain intermediate representations of potentially illegal state. This is both fine and unavoidable, just don't let them leak out of your parser.