Proposal to rename Null->Nullable, Nullable->Nullish

この提案は、Null モジュールと Nullable モジュール、それらが生成する JavaScript の挙動、および ReScript の設定用の機能フラグの可能性に関するものです。ファイル、テスト、エントリポイントが指定されていないため、まずこれらのモジュールを見つけ、提案されている v12–v14 への移行計画を確認してください。実装前に命名と移行の戦略について合意できれば完了です。

I'm perfectly fine with naming we had all this time, but in a new era AI writes most of the code, and I think having a naming parity with JavaScript is more important. For example, this proposal was inspired by the Claude Code session where it was doing bindings for napi-rs and it used Nullable.t type thinking that its value is null, like in TS. I had to correct it myself, saying that it should use a more correct Null.t type.

Additionally, Null.t by itself is already confusing, making me subconsciously think that it's just a null value type.

Proposed migration plan

Since this is a very big breaking change and updating both modules at the same time will be an issue because it might silently change generated JavaScript code, I think we must approach it in multiple major releases. And because of this, start earlier. Here is the proposed plan.

1. In next v12 minor we deprecate Nullable module in favor of the new Nullish module
2. In v13 we we remove Nullable in favor of Nullish
3. In v14 we deprecate Null module in favor of Nullable - I think there should be a whole major version without Nullable module to prevent issues during migration

To accelerate the change, we can add a feature flag in ReScript config which would allow us to explicitly force the migration: Null->Nullable, Nullable->Nullish

Let me know what you think.