Update typing for TypeScript 1.7 "polymorphic this typing"

[benlesh]

Thanks to @trxcllnt for pointing this out:

http://blogs.msdn.com/b/typescript/archive/2015/11/30/announcing-typescript-1-7.aspx

Basically, since our operators can return different types due to the snazzy lift architecture, we should be returning : this and not : Observable<T> from them.

... this might get a little nutty when it comes to types that change the generic output, though. map for example. Observable<T>.prototype.map((x: T) => R): Observable<R> would be Observable<T>.prototype.map((x: T) => R): this ... I'm not sure hot TypeScript will suss out that this should be generically typed to R and not T...

[kwonoj]

I was interested in this topic bit while ago when introduced 1.8 dev of typescript in RxJS, hit couple of roadblocks at the moment.

: lift returns new instance
lift does create new instance and returns it

lift(): Observable {
 ...
 return new Observable();
}

causes compiler complains about type inferences like Type 'Observable' is not assignable to type 'this'

polymorphic this requires to return self, such as

public add(operand: number) {
    this.value += operand;
    return this;
}

: polymorphic this works with instance method
most of operator declaration in Observable is based on property stubs for modularity, such as

switch: <R>() => Observable<R>

can't be converted to return this

switch: () => this;

: type transformed output
copy-paste from original comment.

Observable<T>.prototype.map((x: T) => R): Observable<R> would be
Observable<T>.prototype.map((x: T) => R): this

Still, I'm feeling this is great feature and would explore way to adopt into RxJS.

bit more details about polymorphic this can be found at https://github.com/Microsoft/TypeScript/wiki/What%27s-new-in-TypeScript#this-typing

[kwonoj]

Note : it is completely possible I'm not fully knowledgeable about this feature and above comment could be wrong for some aspect. Above comments are based on my initial exploration.

[benlesh]

That's too bad. I'll close this for now, then. And perhaps file an issue with TypeScript for our specific use case/needs.