title: How RxJS pipe works (lettable operators)
body: <p>This is actually note to self but it could be useful for others.</p>
  
  <p>So, here's 2 pieces of code and my question in gitter:</p>
  
  <p>What's the difference between those two?:</p>
  
  <pre><code>    @Effect()
      loadRegistrationsFailed$: Observable&lt;Action&gt; = this.actions$
          .ofType(registrations.LOAD_FAIL)
          .pipe(
              map(
                  action =&gt;
                      new ShowErrorDialogAction({
                          correlationId: new Guid(),
                          title: "Server is unreachable",
                          message:
                              "Can't load user registrations. Can't connect to the server"
                      })
              )
          );
  ```
  and
  ```
      @Effect()
      loadRegistrationsFailed$: Observable&lt;Action&gt; = this.actions$
          .ofType(registrations.LOAD_FAIL)
          .pipe(action =&gt;
              of(
                  new ShowErrorDialogAction({
                      correlationId: new Guid(),
                      title: "Server is unreachable",
                      message:
                          "Can't load user registrations. Can't connect to the server"
                  })
              )
          );
  </code></pre>