> What is ValidatedData? A subset of the Data that is valid?
Usually, but not necessarily. `validate()` might add some additional information too, for example: `validationTime`.
More often than not, in a real case of applying algebraic data type & "Parse, don't validate", it's something like `Option` or `Result`, borrowing Rust's names. `Option` & `Result` expand the possible return values that function can return to cover the possibility of failure in the validation process, but it's independent from possible values that `ValidatedData` itself can contain.
> The way I see it is you use ‘validate’ when the format of the data you are validating is the exact same format you are gonna be working with right after, meaning the return type doesn’t matter.
The main point of "Parse, don't validate" is to distinguish between "machine-level data representation" vs "possible set of values" of a type and utilize this "possible set of values" property.
Your "the exact same format" point is correct; oftentimes, the underlying data representation of a type is exactly the same between pre- & post-validation. But more often than not "possible set of values" of `ValidatedData` is a subset of `Data`. These 2 different "possible set of values" are given their own names in the form of a type `Data` and `ValidatedData`.
This distinction is actually very handy because types can be checked automatically by the (nominal) type system. If you make the `ValidatedData` constructor private & the only way to produce is function `ValidatedData validate(Data)`, then in any part of the codebase, there's no way any `ValidatedData` instance is malformed (assuming `validate` doesn't have bugs).
Extra note: I forgot to mention the "Parse, don't validate" article implicitly implies a nominal type system, where 2 objects with equivalent "data representation" doesn't mean it has the same type. This differs from Typescript's structural type system, where as long as the "data representation" is the same, both object are considered to have the same type.
Typescript will happily accept something like this because of structural
type T1 = { x: String };
type T2 = { x: String };
function f(T1): void { ... }
const t2: T2 = { x: "foo" };
f(t2);
While nominal type systems like Haskell or Java will reject such expressions
class T1 { String x; }
class T2 { String x; }
void f(T1) { ... }
// f(new T2()); // Compile error: type mismatch
Because of this, the idea of using type as a "possible set of values" probably felt unintuitive to Typescript folks, as everything is just stringly-typed and different type felt synonymous with different "underlying data representation" there.
You can simulate this "same structure, but different meaning" concept of nominal type system in Typescript with some hacky workaround with Symbol.
> The return type implies transformation – a write operation per se, whereas validation is always a read operation only
Why does the return type need to imply transformation and why is "validation" here always read-only? No-op function will return the exact same value you give it (in other words, identity transformation), and Java & Javascript procedures never guarantee a read-only operation.