Tuples as types for rest ...arguments

[g162h3]

This feature could be used in observable-like generic classes/interfaces:

interface EventSource<Args extends any[]> {
 observe(listener: (...args: Args)): void;
 map<T>(mapping: (...args: Args) => T): EventSource<[T]>;
}

Currently the only way I know of is to fallback to any.

[RyanCavanaugh]

Could you write up a bit longer explanation of how you'd want this to work? For example, if the declaration of map was instead map<T>(x: T, mapping: (...args: Args) => T, y: T): EventSource<[T]>;, what would the result be?

[sandersn]

This sounds a lot like #4130. I have a proposal up at #5296, but I think it's currently too narrow for your use. I am definitely interested in seeing a good real-world explanations of how you (and others) would like it to work.

[d180cf]

Ryan Cavanaugh (@RyanCavanaugh), I suppose you meant this signature: mapping: (...args: Args, y: T) => T. If es6 doesn't allow named args after ...args, then tsc can do the same.

[eggers]

Another use case for this is to properly declare the spread function. Something like this:

interface Promise<T> {
 spread<U>(fulfilledHandler: (...values: T & any[]) => U | Promise<U>): Promise<U>;
 spread<U>(fulfilledHandler: (...values: [T]) => U | Promise<U>): Promise<U>;
}