Custom array index types

Search Terms

custom array index type

Suggestion

Arrays are indexed by integers, cool. But they could also be indexed by something equivalent to an integer, like an enum-like class or wrapper around a number. For example:

class PlayerNumber { constructor(readonly n: number); valueOf() { return this.n; } }
let currentTurn = 3;
const players: Player[];
let currentPlayer = new PlayerNumber(4);

// correct
players[+currentPlayer]
// oops, typo, but no protection
players[+currentTurn]

Imagine we had a TypedArray<K extends { valueOf(): number }, V> that in all respects was like Array, except all array indices in any function signature were of type K, and all arrays in any function signature were instead TypedArray<K, ...>. And the + wouldn't be necessary. Then:

// correct AND AWESOME
players[currentPlayer]
// compiler error
players[currentTurn]

Use Cases

I am writing a game framework in TypeScript and there are just lots of arrays everywhere indexed by players, token types, round number, etc and it's easy to mess up which index goes where.

Checklist

My suggestion meets these guidelines:

• 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, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

I'm unclear on the last bullet. There is a bit of "operator overloading" involved in the proposal as suggested. I am not sure if that is disqualifying and/or if there is an alternative that would work.

Start by examining the proposal's TypedArray<K, V> model and how TypeScript currently types array indexing and valueOf-based expressions. Determine whether the requested type safety can fit existing type-system rules without runtime changes; done means a concrete, accepted design with corresponding type-checking behavior and tests.