get_elem_at_index(array, index)
"In Idris, a length-indexed vector is Vect n a (length n is in the type), and a valid index into length n is Fin n ('a natural number strictly less than n')."
Similar tricks work with division that might result in inf/-inf, to prevent them from typechecking, and more subtle implications in e.g. higher order types and functions
Type-Driven Development with Idris[1] is a great introduction for dependently typed languages and covers methods such as these if you're interested (and Edwin Brady is a great teacher).
[1] https://www.manning.com/books/type-driven-development-with-i...
(This isn’t to say it’s always wrong, but that having it be an error state by default seems very reasonable to me.)
https://www.stroustrup.com/Concept-based-GP.pdf
{
Number<unsigned int> ii = 0;
Number<char> cc = '0';
ii = 2; // OK
ii = -2; // throws
cc = i; // OK if i is within cc’s range
cc = -17; // OK if char is signed; otherwise throws
cc = 1234; // throws if a char is 8 bits
}Like how clojure basically uses maps everywhere and the whole standard library allows you to manipulate them in various ways.
The main problem with the many type approach is several same it worse similar types, all incompatible.
The way I've thought about it, though, is that it's possible to design a program well either by encoding your important invariants in your types or in your functions (especially simple functions). In dynamically typed languages like Clojure, my experience is that there's a set of design practices that have a lot of the same effects as "Parse, Don't Validate" without statically enforced types. And, ultimately, it's a question of mindset which style you prefer.
The real world often changes though, and more often than not the code has to adapt, regardless of how elegant are systems are designed.
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-...
```
impl Add for NonZeroF32 { ... }
impl Add<f32> for NonZeroF32 { ... }
impl Add<NonZeroF32> for f32 { ... }
```
What type would it return though?
dang•1h ago
also:
Parse, Don’t Validate – Some C Safety Tips - https://news.ycombinator.com/item?id=44507405 - July 2025 (73 comments)
Parse, Don't Validate (2019) - https://news.ycombinator.com/item?id=41031585 - July 2024 (102 comments)
Parse, don't validate (2019) - https://news.ycombinator.com/item?id=35053118 - March 2023 (219 comments)
Parse, Don't Validate (2019) - https://news.ycombinator.com/item?id=27639890 - June 2021 (270 comments)
Parsix: Parse Don't Validate - https://news.ycombinator.com/item?id=27166162 - May 2021 (107 comments)
Parse, Don’t Validate - https://news.ycombinator.com/item?id=21476261 - Nov 2019 (230 comments)
Parse, Don't Validate - https://news.ycombinator.com/item?id=21471753 - Nov 2019 (4 comments)
(p.s. these links are just to satisfy extra-curious readers - no criticism is intended! I add this because people sometimes assume otherwise)