example-rx-fp.component.ts

import { Component, OnInit } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { Post } from '../model';
import { Observable } from 'rxjs/Observable';
import { Subject } from 'rxjs/Subject';

import "rxjs/add/observable/combineLatest";
import "rxjs/add/observable/merge";
import "rxjs/add/operator/map";
import "rxjs/add/operator/switchMap";

@Component({
  selector: 'app-example-rx-fp',
  template: `
  <button (click)="goodCommentsClickStream.next()">Fetch good comments</button>
  <button (click)="badCommentsClickStream.next()">Fetch bad comments</button>
  <pre>{{ (comments | async) | json }}</pre>
  `,
  styles: []
})
export class ExampleRxFpComponent implements OnInit {
  private url = "https://jsonplaceholder.typicode.com/posts";  

  comments: Observable<Comment[]>;
  goodCommentsClickStream: Observable<any> = new Subject<any>();
  badCommentsClickStream: Observable<any> = new Subject<any>();

  constructor(private httpClient: HttpClient) { }

  ngOnInit() {
    const dataStream = this.httpClient.get<Post[]>(this.url);
    const goodComments = this.goodCommentsClickStream.switchMap(
      () => this.httpClient.get<Comment[]>(this.goodCommentsUrl)
    );
    const badComments = this.badCommentsClickStream.switchMap(
      () => this.httpClient.get<Comment[]>(this.badCommentsUrl)
    );
    this.comments = Observable.merge(
      goodComments,
      badComments
    );
  }
  
  private goodCommentsUrl = "https://jsonplaceholder.typicode.com/comments?postId=1";  
  private badCommentsUrl = "https://jsonplaceholder.typicode.com/comments?postId=2";  
}

Than you for an example for ngPoland 2017.