Rename Nullable to Nullish

[DZakh]

I find the Nullable module name very confusing because usually, it's 'a | null, not 'a | undefined | null It's not super critical to me, but I think for ReScript's future, it will be better to rename it to Nullish early on (at least do it in rescript-core).

According to https://tc39.es/ecma262/ nullish is precisely the values that can be both null and undefined.

Because right now for users coming from JS/TS there's the following situation:

TS type	Expectation	Reality
T | undefined	optional<'a>	option<'a>
T | null	nullable<'a>	null<'a>
T | undefined | null	nullish<'a>	Js.Nullable.t<'a>

I think that we shouldn't do anything with option and null; that's the language specific. But the problem is that usually nullable means T | null instead of T | undefined | null.

[DZakh]

What do you think?

[cometkim]

The word "nullish" has never been adopted by the ecosystem. I think that would be a little strange.

The words I see the most are "maybe" or "optional".

[CarlOlson]

The word "nullish" has never been adopted by the ecosystem. I think that would be a little strange.

With the nullish coalescing operator I'd expect it to be pretty understandable.

I don't think this is a big issue since option is used much more frequently. However, I have always found Nullable to be a strange name.

I think it is reasonable that some of these modules could be removed and made more consistent:

let _: Js.null<'a> = Js.null
let _: Js.Null.t<'a> = Js.Null.empty

let _: Js.undefined<'a> = Js.undefined
let _: Js.Undefined.t<'a> = Js.Undefined.empty
let _: option<'a> = None

let _: Js.null_undefined<'a> = Js.Null_undefined.null
let _: Js.null_undefined<'a> = Js.Null_undefined.undefined
let _: Js.Nullable.t<'a> = Js.Nullable.null
let _: Js.Nullable.t<'a> = Js.Nullable.undefined

And being able to easily pattern match would be big.

[cometkim]

It is called as nullable for even TypeScript people: https://www.typescriptlang.org/docs/handbook/utility-types.html#nonnullabletype

Maybe we can remove something inconsistent, but I don't think the "nullable" is not a problem