[Help] Function that returns a polymorphic instance of a Class

So I have a class `DataLoader` - you may have recognized the [dataloader module](https://github.com/facebook/dataloader).

I have a function that returns a new DataLoader with default options.
```
const myFn = (batchLoadFn) => new DataLoader(batchLoadFn, { cache: false });
```

I tried to type it as
```
const myFn/*: (fn: any) => Dataloader */ = ... ;
```

But it won't work. Any idea on how to achieve the right typing?