Deprecate homomorphic mapped type over primitive

🔍 Search Terms

homomorphic mapped primitive

✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

I would like homomorphic mapped types over primitives to emit a warning if the base type is not an object type. An explicit conditional type Foo<T> = T extends object ? { [K in keyof T]: ...} : T can be used to recover the pre-existing behavior. This should be mentioned in the warning message.

Homomorphic mapped type over primitives has surprising behavior that bears mention in the FAQ:

• This mapped type returns a primitive type, not an object type.
  • Mapped types declared as { [ K in keyof T ]: U } where T is a type parameter are known as homomorphic mapped types, which means that the mapped type is a structure preserving function of T. When type parameter T` is instantiated with a primitive type the mapped type evaluates to the same primitive.

This behavior is surprising since:

1. It is not referentially transparent; that is, replacing a type parameter with its value does not result in the same type
2. Although a mapped type may be assignable to the original type, it may specifically not be, e.g. {[k in keyof T]: never} whereas it always is in the case of primitives.
3. The expression keyof T has surprising behavior on primitive types, differing substantially from object types.
4. The syntax of a mapped type visually suggests an object-like type, which is incorrect when mapping over null.

📃 Motivating Example

type NullValued<X> = {[k in keyof X]: null};
type Okay = NullValued<{x:1, y:2}>; // {x:null, y:null}
type Oops = NullValued<number>; // oops! This is `number`, which is likely not what you expected

💻 Use Cases

1. What do you want to use this for?
2. What shortcomings exist with current approaches?

• You have to know about homeomorphic mapped types and anticipate this corner case.

3. What workarounds are you using in the meantime?

• A type guard type Foo<T extends object> = { [K in keyof T]: ...} guards against the current behavior.
• A conditional type Foo<T> = T extends object ? { [K in keyof T]: ...} : T makes the current behavior explicit.

動機となる NullValued の例と issue 内の FAQ の記述から始めてください。実装ファイルもテストも指定されていません。homomorphic mapped types が primitive bases に対して要求された警告を生成し、その警告が明示的な conditional workaround を説明し、さらにその意外な挙動が FAQ に記録されていれば完了です。