Allow mapped tuples to have behaviour based on index

Search Terms

mapped tuples index

Suggestion

I want to be able to create a tuple from function parameters and have all elements in the tuple optional other than the firstK

Use Cases

I am trying to create a curry implementation that allows you to curry any number of parameters. The first parameter must be specified but all other params are optional.
Would be good to be able to remove the last param as well.

Examples

something along the lines of

type OptionalExceptFirst<T> = {
    [K in keyof T]:isFirst ? ? T[K] : T[K];
};
type AllExceptLast<T> = {
    [K in keyof T]:isLast ? never : T[K];
};

The syntax will need some work hopefully the intended functionality is clear.

Checklist

My suggestion meets these guidelines:

• [x ] This wouldn't be a breaking change in existing TypeScript/JavaScript code
• [ x] This wouldn't change the runtime behavior of existing JavaScript code
• [ x] This could be implemented without emitting different JS based on the types of the expressions
• [ x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• [ x] This feature would agree with the rest of TypeScript's Design Goals.

The issue names no files, tests, or entry points, so first review how mapped tuples currently behave and how TypeScript represents tuple positions. Clarify the intended rules for optionality based on the first and last indexes before proposing implementation and tests; done would require an agreed design plus validated compiler behavior.