More strong Promise<T> in lib.es6.d.ts

I think it would be good to be able to define the type of error in Promise type. I suggest promise type to be something like this:

interface Promise<T,U> {
    /**
    * Attaches callbacks for the resolution and/or rejection of the Promise.
    * @param onfulfilled The callback to execute when the Promise is resolved.
    * @param onrejected The callback to execute when the Promise is rejected.
    * @returns A Promise for the completion of which ever callback is executed.
    */
    then<TResult>(onfulfilled?: (value: T) => TResult | PromiseLike<TResult>, onrejected?: (reason: U) => TResult | PromiseLike<TResult>): Promise<TResult>;
    then<TResult>(onfulfilled?: (value: T) => TResult | PromiseLike<TResult>, onrejected?: (reason: U) => void): Promise<TResult>;

    /**
     * Attaches a callback for only the rejection of the Promise.
     * @param onrejected The callback to execute when the Promise is rejected.
     * @returns A Promise for the completion of the callback.
     */
    catch(onrejected?: (reason: U) => T | PromiseLike<T>): Promise<T>;
    catch(onrejected?: (reason: U) => void): Promise<T>;

    [Symbol.toStringTag]: string;
}

Or at least have a Promise<T, U> type which extends Promise<T>.

[NoelAbrahams]

This is another good use-case for implementing #2175.

interface Promise<T, U = Error> {
 //  ...
}

[hrajchert]

I agree the ideal solution would be to have typed exceptions and then being able to correctly type Promises… but seeing that it may take a looooong time to have typed exceptions, I created a library that works likes promises but with a subtle difference that allows me to have typed errors and a pretty good error inference. Been using it on production for more than 6months and I’m really happy with the results.

The whole problem arises because we can’t forbid a function from throwing, and we cant know the type that it’s thrown, so what I did is favour the idiomatic way of returning an error, which is returning a rejected “Promise” and if for some reason a function throws, i put the error inside an UnknownError class, with an unknown property.

Here’s the library https://github.com/ts-task/task and here is a video explain in it with more detail https://www.youtube.com/watch?v=T7O1T1wmw00