the type `null | true | false` is different from `true | false`, a type checker can assert that you handle the `null` case before using a function that wants a boolean. This is how rust handles it (with the Option<T> type).

In particular, you may want this to signify “I dont know” with NULL, just as in SQL. See the Wikipedia page for ternary logic for more info.

Note that 3VL (ternary logic/3 valued logic) is one of the more criticized aspects of SQL.

3VL is a gigantic leap of of complexity over 2VL (boolean). The number of possible operations is a lot higher and the choice of operations is not standardized. To quote wikipedia: "In two-valued logic there are 2 nullary operators (constants), 4 unary operators, 16 binary operators, 256 ternary operators" And we all agree which of those 16 is named what. "In three-valued logic there are 3 nullary operators (constants), 27 unary operators, 19683 binary operators, 7625597484987 ternary operators" There is no standard meaning of the 3'rd value and of the operations involving it.

That is not to say that 3VL isn't useful. I believe the reason for the criticism of 3VL in SQL it that the system made those choices for you and those choices are not always intuitive. Therefore a null in one attribute in one relation might mean something very different from a null in another attribute in another relation.

Yes, so it's just about returning a (Maybe = Just bool | Nothing). No null needed.

exaclty, whether you call it 'Nothing' or 'null' does not matter, as long as it represents a distinct type than 'true' or 'false'

Null for me is a null pointer, and thus there's a difference. Anyway, if that's all you want than Haskell, etc. definitely handle that case.

The word "null" means "missing value" in e.g. SQL and most data science / analytics contexts. It's unfortunately an overloaded term with two different meaning. Conflating the two things is where we get the egregious mistake of using null pointers to represent null data!

That doesn’t make sense. `null | false | true` is not equivalent to `Option<bool>` or `Option<false | true>`. Just like `zero | one | two` is not equivalent to `Option<one | two>`.

Assuming that `true | false` is equivalent to something like `enum { true, false }`.

> Just like `zero | one | two` is not equivalent to `Option<one | two>`

Isn't it ? both cases represent a type than can express 3 variants

It is equivalent in the amount of information you can encode, but not in how you can use it.

Classical example is wrapping multiple times: Option<Option<one | two>>. If you have null | null | one | two, well... that just boils down to null | one | two.

Those two types are indeed equivalent by themselves. The problem comes when you want to store them somewhere nullable. `zero | one | two | zero` flattens back down to `zero | one | two`, you can't distinguish between the two zero/null cases.

On the other hand, `Option<Option<one | two>>` allows you to distinguish between None and Some(None).

This makes union types unsound in the presence of type parameters/generics.

TypeScript supports both anyway, because Hejlsberg cares more about being able to type existing JS antipatterns than about providing a sound type system.

> This makes union types unsound in the presence of type parameters/generics.

I'm not sure if "unsound" is a good adjective here. There are cases where this is actually desired behaviour and the rules can definitely be "sound".

For example, I might want to know what errors can appear, but not care where they come from. So `ErrorA | ErrorB` is what I want to see, not some nested structured that allows me to differentiate where ErrorA came from in case that there are multiple possible options.

I did not catch from you comment if you knew, but "sound" and "unsound" are specific concepts in type theory, and they are binary properties. A system either is or is not sound.

Yeah I know that.

So: > This makes union types unsound in the presence of type parameters/generics.

Sounds a bit strange to me. Why would union types + type parameters be generally unsafe? I doubt that that's true.

Sound type system is not one of the design goals of TypeScript

https://effectivetypescript.com/2021/05/06/unsoundness/

No.

There is an isomorphism between them, but they aren’t equivalent since for one you will have to match on the `Option` first in order to see whether it is `None` or `Some(Next)` and then inspect `Next` (if `Some(…)`).

Same reason that `Nothing | Pointer` is not equivalent to `Option<Pointer>`. And it makes a huge practical difference, since the first type allows for “nothing-pointer dereference” while the second one does not.

> And it makes a huge practical difference, since the first type allows for “nothing-pointer dereference”

That's not how strict TypeScript works. If you have a nullable you'll need to prove to the compiler first that it is not currently null before dereferencing.

Here’s what I originally replied to:

> the type `null | true | false` is different from `true | false`, a type checker can assert that you handle the `null` case before using a function that wants a boolean. This is how rust handles it (with the Option<T> type).

If the variant `null` here is handled specially in general in TS then yeah, I was wrong. However, I was mostly replying to the part about “this is how Rust handles it”.