Ggtcf

I'd like this merge method to have a return type of a union of all the different array types passed into it.

type AnyIterable<T> = Iterable<T> | AsyncIterable<T>

async function* merge<T>(...iterables: Array<AnyIterable<T>>) 
 for (const iterable of iterables) 
 yield* iterable
 


merge([1, 2, 3], ['1', '2', '3'])
// Argument of type 'string[]' is not assignable to parameter of type 'AnyIterable<number>'.
// Property '[Symbol.asyncIterator]' is missing in type 'string[]' but required in type 'AsyncIterable<number>'.ts(2345)

But I get that error. Obviously that's expected but I don't know how to type this to get the values from the array of iterables.

Hmm, I think people usually want errors like this to happen, since heterogeneous arrays are less common than homogeneous arrays, and type inference that always widened until it worked would not catch actual errors.

If you want to work around this, it's possible the go the other direction: force the type inference to succeed, and then calculate the value of T from it if you need it:

async function merge<I extends Array<AnyIterable<any>>>(...iterables: I) 
 for (const iterable of iterables) 
 


type UnArrayAnyIterable<A extends Array<AnyIterable<any>>> = 
 A extends Array<AnyIterable<infer T>> ? T : never;

Now this succeeds:

merge([1, 2, 3], ['1', '2', '3']); // I inferred as the tuple `[number[], string[]]`

And if you want T you can use UnArrayAnyIterable:

declare function foo<I extends Array<AnyIterable<any>>>(...iterables: I): UnArrayAnyIterable<I>;
const ret = foo([1, 2, 3], ['1', '2', '3']); // string | number

Okay, hope that helps. Good luck!