Comment by jerf

"This is just validation that is using the type system to indicate the validation has already occurred. I think the real point of “parse, don’t validate” is to make the type system give you structural guarantees that couldn’t exist otherwise (e.g. always having a first/last element in the NonEmpty example from the original article)."

It's the same thing. In the latter case, something has validated that your NonEmpty has a first and a last element. It's all validation before you stick it in a type that asserts that the validation is guaranteed to have occurred so every function receiving it doesn't need to do it itself.

Any non-trivial use of a type system will involve making guarantees the type system itself can not actually express [1]. There's nothing wrong with saying "this is a valid email in accordance with my standards" in a type. Merely using the type system to assert "I have some sort of value in the name and host fields" is valid but a degenerate use. "struct Email { name: Name, host: Hostname }" is an even stronger use of the type system, where Name and Hostname are themselves values you can only get by passing some incoming string through a validation process. Asserting that these things exist is just the most basic check possible, but your type still permits {name: "\0\0\0\0\0\0", host: "!"}, whereas under my definition, assuming that Name and Hostname are reasonably defined, that value will not be ever be something that can be witnessed.

In fact in general, while I don't absolutely rigidly apply this, especially in smaller script-like programs, when a "string" appears in my strong types that specifically means "this has unbounded contents". It's an appropriate type for "stuff I got off a network" or "stuff a user typed". What stuff? Don't know. Haven't checked it yet. When I do it'll get a more specific type like a Username or DecodedUTF8String or something else. Thanks to people using way too many "strings" and "ints" in the world I have to constantly explain to my LLM that I want stronger types. I'm yet to find the invocation to put into my CLAUDE.md or equivalent to get it to do it right the first time consistently.

[1]: With a wistful stare into the distance acknowledging the theoretical utopia of dependent types... but it doesn't seem to be coming down from "theoretical" any time soon.