[demurgos]

A situation where I needed this were typings for Cycle.js a couple years ago. At this time, you could configure it to use the observable library of your choice but it was hard to type. In particular the observable could be a `Stream from "xstream"` or `Observable from "rxjs"`

This lib has a method `select` to return an observable list of DOM elements for a given CSS selector. What should be its signature? If using `xstream`, it should be `select(selector: string): Stream<HTMLElement[]>`, and if using `rxjs` it should be `select(selector: string): Observable<HTMLElement[]>`. Let's also assume that it has a second method `windowHeight` returning an observable for the window height (`Stream<number>` or `Observable<number>`).

We can make the lib object generic over the observable implementation, but you need HKTs to type it properly. The reason is that the lib is generic over an already generic type.

Here is an example:

    // Without HKT (regular generic)
    // Problem: both `select` and `windowHeight` return the same type (we lose the HTMLElement[]/number information)
    interface Cycle<Obs> {
      select(selector: string): Obs;
      windowHeight(): Obs;
    }
    // The best we can do is type it as `Cycle<Stream<unknown>>` or `Cycle<rxjs.Observable<unknown>>`.
    
    // With HKTs, using syntax from this lib, you could do:
    interface Cycle<$HktObs> {
      select(selector: string): apply<$HktObs, HTMLElement[]>;
      windowHeight(): apply<$HktObs, number>;
    }
    // This allows to get the precise signatures we wanted (with the right observable impl, without `unknown`)