Note that the division-by-zero example used in this article is not the best example to demonstrate "Parse, Don't Validate," because it relies on encapsulation. The principle of "Parse, Don't Validate" is best embodied by functions that transform untrusted data into some data type which is correct by construction.

Alexis King, the author of the original "Parse, Don't Validate" article, also published a follow-up, "Names are not type safety" [0] clarifying that the "newtype" pattern (such as hiding a nonzero integer in a wrapper type) provide weaker guarantees than correctness by construction. Her original "Parse, Don't Validate" article also includes the following caveat:

> Use abstract datatypes to make validators “look like” parsers. Sometimes, making an illegal state truly unrepresentable is just plain impractical given the tools Haskell provides, such as ensuring an integer is in a particular range. In that case, use an abstract newtype with a smart constructor to “fake” a parser from a validator.

So, an abstract data type that protects its inner data is really a "validator" that tries to resemble a "parser" in cases where the type system itself cannot encode the invariant.

The article's second example, the non-empty vec, is a better example, because it encodes within the type system the invariant that one element must exist. The crux of Alexis King's article is that programs should be structured so that functions return data types designed to be correct by construction, akin to a parser transforming less-structured data into more-structured data.

[0] https://lexi-lambda.github.io/blog/2020/11/01/names-are-not-...

Even the newtype-based "parse, don't validate" is tremendously useful in practice, though. The big thing is that if you have a bare string, you don't know "where it's been". It doesn't carry with it information whether it's already been validated. Even if a newtype can't provide you full correctness by construction, it's vastly easier to be convinced of the validity of an encapsulated value compared to a naked one.

For full-on parse-don't-validate, you essentially need a dependent type system. As a more light-weight partial solution, Rust has been prototyping pattern types, which are types constrained by patterns. For instance a range-restricted integer type could be simply spelled `i8 is 0..100`, or a nonempty slice as `[T] is [_, ..]`. Such a feature would certainly make correctness-by-construction easier in many cases.

The non-empty list implemented as a (T, Vec<T>) is, btw, a nice example of the clash between practicality and theoretical purity. It can't offer you a slice (consecutive view) of its elements without storing the first element twice (which requires immutability and that T: Clone, unlike normal Vec<T>), which makes it fairly useless as a vector. It's okay if you consider it just an abstract list with a more restricted interface.

You can also search for "make invalid states impossible/unrepresentable" [0] to find more info on related practices. See "domain modeling made functional" [0] as a nice example

[0] https://geeklaunch.io/blog/make-invalid-states-unrepresentab...

[1] https://www.youtube.com/watch?v=2JB1_e5wZmU