Comment by dllthomas
"Typing your code forces validations at the boundaries" was too strong because of course you can type your code without actually doing the validations, but you can structure your code such that that won't happen accidentally: https://lexi-lambda.github.io/blog/2019/11/05/parse-don-t-va...
The idea is that checking should be the only way of making a value of the type. That prevents you from forgetting to check when you turn some broader type (say, string) into the more narrow one (date, in this case).
> "Typing your code forces validations at the boundaries" was too strong because of course you can type your code without actually doing the validations
Yeah, of course you can cheat the typechecking in the code at the boundary in several ways, or convert from wire format to internal types in a way which plugs in type-valid defaults for bad data rather than erroring, or just use too-broad internal types to start with (you can have "stringly-typed code"), and fail to help the problems. But if you use the types that make sense internally for what the code is doing, than conversion including validation at the boundary becomes the path of least resistance in most cases. "Forces" is not strictly true, but my experience is that adding types does create a strong push for boundary validation.