No, not true!
As the author correctly states earlier in the post, unions are not an exclusive-or relation. Unions are often made between disjoint types, but not always.
This becomes important when T itself is nullable. Let's say T is `U | null`. `Option<Option<U>>` in Rust has one more inhabitant than `U | null | null` in TypeScript - `Some(None)`.
Union types can certainly be very useful, but they are tricky because they don't compose like sum types do. When writing `Option<T>` where T is a generic type, we can treat T as a totally opaque type - T could be anything and the code we write will still be correct. On the other hand, union types pierce this opacity. The meaning of `T | null` and the meaning of code you write with such a type does depend on what T is at runtime.
Your last sentence put the whole argument even better in place in my head, it depending on the runtime is both a blessing and a curse and is what let's us change function signatures in a backwards compatible way, while also what hinders our ability to reason/encode stuff in it statically.
[1] I think part of the misunderstanding here between the "two camps" is that some people work on systems that are a closed world. You know and control everything, so an all-encompassing type system that can evolve with the universe itself makes the most sense. Hickey on the other hand worked/works mostly in an area where big systems developed by completely different entities have to communicate with each other with little to no coordination. You can't willy nilly refactor your code and just trust the compiler to do its job, this is the open sea. Also, I think this area is a bit neglected by more modern/hyped languages, e.g. the dynamicism that the JVM has is not really reproduced anywhere anymore?
I've always found this to be a silly argument. In both cases, the problem can be solved by interface versioning. If you have f : T → Maybe T and you should like to change the type, just create f_v2 : Maybe T → T assign f x = just (f_v2 (just x)). The real shame is that most languages do not support interface versioning as a proper feature. There should really be some way to use a package and declare that you want to use v1/2 of the interface, then subsequently package.f would either be f or f_v2. You would eventually have to change the code that deals with f if you want to stay up to date, but realistically you should remove redundant error checking code anyway so you aren't much better off.
You could also frame it as a tooling problem. Given that the transformation between types is trivial, you should be able to write a program that automatically updates code to newer versions. It seems like this is a relatively niche issue in either direction (union types not being disjoint, vs having to change code that deals with options), but its a more solvable problem in the second case.
You can do this:
const user = await fetchUserDTO(123);
// user is Result<UserDTO, Error>
if(!isOk(user)) { something(user.err); return; }
user.name; // user is narrowed to UserDTO
export const FailSym: unique symbol = (globalThis as any).____FailSym ?? Symbol('FailSym');
(globalThis as any).____FailSym = FailSym;
export type Result<T, E> = T | {
[FailSym]: true,
err: E
}
export function Ok<T>(val: T): Result<T, never> { return val; }
export function Fail<E>(err: E): Result<never, E> { return {[FailSym]: true, err}; }
export function isOk<T, E>(val: Result<T, E>): val is T { return !(typeof val == 'object' && val != null && FailSym in val); }
export function isFail<T, E>(val: Result<T, E>): val is Result<never, E> { return typeof val == 'object' && val != null && FailSym in val; }
export function resultify<T, E>(promise: Promise<T>): Promise<Result<T, E>> {
return promise.then(Ok, err => Fail(err as any));
}Could you give an example of this? The only case I can imagine is a union null <=: T, T | null. This would cause unexpected behaviour, i.e. a if T = (U | null), the function orElse : T | null → T → T would always return the second argument if the first is null, but this is perfectly consistent with the fact that T itself is a union type. The idea that it should do anything different is presuming that the null assigned to T is different from the null assigned to T | null (that union types are disjoint). But this is an invalid assumption. It's still a validly typed program that obeys a consistent set of rules. It's just that there is no tagging in the union type (you don't know if a member comes from the left or right side). This is only an issue if you write code expecting the union to work like a tagged union.
Just to point that this part is very literally true. They compose perfectly well, but they don't compose on the same way that tagged unions do. Tagged unions compose by function abstraction, untagged ones compose by function dispatching.
Maybe one can argue that untagged unions compose in less useful ways. But I've never seen this argument made.
Similar to this, I proposed inequality types for TS. They allow constraining a number range. For example, it is possible to have the type "a number that is larger than 1". You can combine them using intersection types, forming intervals like "a number between 0 and 1". Because TS has type narrowing via control flow, these types automatically come forward if you do an "if (a<5)". The variable will have the type "(<5)" inside the if block.
You can find the proposal here [1]. Personally I think the effort for adding these isn't worth it. But maybe someone likes it or takes it further.
`any`, however, is not `top`, it is `break_the_type_system`. The top type in TS is `unknown`.
You don’t even know how to query it to find anything out about it.
But you could pass it to someone else.
When someone asks you for a completely unconstrained object, the type is “any”.
It’s technically the same type from two perspectives.
(Not saying this extreme version of the concepts are how they are implemented. Never had a chance to use such types before.)
1. As a nit-pick "unconstrained object" is not best modeled by `unknown` because that includes non-objects as well, there's better types to use for that
2. Someone asking you for any unconstrained data would also be `unknown`
`any` is not a type at all, it is an annotation to disable the type system
Scala3 is the only programming language to implement both AFAIK.
C# has a proposal to add both unions and disjoint unions: https://github.com/dotnet/csharplang/blob/main/proposals/Typ...
OCaml has polymorphic variants which are open disjoint unions.
Kotlin is looking to add union types for errors: https://youtrack.jetbrains.com/issue/KT-68296/Union-Types-fo...
I believe Java's checked exceptions behave somewhat like union types.
Do you mean implemented with enums? Enums themselves are not a type. They are a mechanism for value generation, providing automatic numbering (hence enumeration) for constants. Indeed, they, like all values, are ultimately represented by a type, but that type can range from something like a simple integer or something more complex like a tagged union (typically with the generated value being the tag) with different ecosystems favouring different type approaches.
I think they might have meant "entities" instead of "entries?"
The term "diagonal identity" seems to be non-standard as well?
I didn't feel satisfied with the explanation in TFA.
Are these even types? I always mentally filed closures under "implementation detail of nested functions".