Type compatibility/can assign

[carloszimm]

Forgive me if I have understood wrong (the functionality) but the following code shouldn't work without compiler problems?

assertType.isTrue(true as CanAssign<any, number>);

If the order is inverted, it works fine. Also, the following code apparently compiles:

let a: any = 4;
let b: number = a;

let d: number = 3;
let e: any = d;

StackBlitz print:

Thank you :)

[unional]

Hi, yes, you are technically correct. TypeScript has some weird behavior around any.

Besides meaning any type, or union of all types, or the superset of all types, any is also used as a "type checking silencer", allowing user to tell the compiler that "hey, don't worry about the type of this".

That's why you can do const x: number = 'a' as any.

The CanAssign<A, B> type was checking more like on the set theory, which any cannot assign to number.

In fact, the CanAssign<any, number> should really return boolean, instead of false, as "some branch in the union (of any) can be assigned to number, while some cannot".

This type will be updated in the near future, but it will be a breaking change.

[carloszimm]

The CanAssign<A, B> type was checking more like on the set theory, which any cannot assign to number.

As I suspected 😅
Anyway, while the update doesn't come up, a possible workaround can be to check if either operand is the type any, so there will be no need to proceed with the type checking. Also, it seems to be working properly when incorporated inside a class for instance (so the workaround should work):

[unional]

Yes. The IsAny check can be applied. Many of the newer types such as IsEqual is doing that kind of checks.

CanAssign can be improved with that too.

For the inside a class/object, yes. That's the original purpose of the type and that is handled differently.

I'm working on a new way to handle branching to maximize flexibility. CanAsssign can use some updates when I get to it.

btw, if you want to check for CanAssign between two types, you can do testType.canAssign<A,B>(true | false).

[carloszimm]

I wasn't aware that testType had a canAssign (the documentation apparently didn't mention it yet). Anyway, is it supposed to behave similarly to assertType (with the compiler complaining about the wrong type) or it should return only a boolean? If the later, why the following snippet is complaining? :(

Btw, I suppose testType is one of the types you mentioned that is making those checks, as it works correctly for the following test:

console.log(testType.canAssign<number, any>(true)); //--> true
console.log(testType.canAssign<any, number>(true)); //--> true

[unional]

I wasn't aware that testType had a canAssign (the documentation apparently didn't mention it yet).

Um, thanks for pointing it out. I thought I have the doc updated. Seems like I have missed it.

It is in the docs under the /testing folder,
and it has JSDocs and unit tests describing each function.

It is mainly for used during testing. The return value is merely a container of the type under inspection, as all the check are done with pure type anyway (the same for assertType.isTrue(true ...), the return value can only be true 😄 )

For runtime check, currently the canAssign<T>(): (subject: S) => CanAssign<S, T> is the one you can use.