I saw a Monad in Dart Stream

I knew I had read it, briefly, in one or two of the articles about functional programming before. Or if I type something about "Functional Programming", "Reactive X", and "Dart Stream" into DuckDuckGo, the first page will be full of them.

It is when I wrote these lines of code...

Rx.combineLatestList(
  [_siteInfoController.stream, _productInfoController.stream])
        .flatMap((List<String?> p) {

      if (p.first == null) {
        return Stream.value(_backToCheckAddress());
      } else if (p.last == null) {
        return Stream.value(_backToSiteProducts(p.first!));
      } else {
        final siteInfo = getIt<TaskManager>()
            .runTask(name: 'getting site', task: getSiteInfo(p.first!));
        final productInfo = getIt<TaskManager>()
            .runTask(name: 'getting product', task: getProductInfo(p.last!));
        
          return Rx.zipList([siteInfo.asStream(), productInfo.asStream()])
            .map<Component>((p) {
          return _siteAndProductInfo(p);
        });
      }
    });

Then I realized I am dealing with Monad, by writing the `.flatMap`. Instead of ending up with a `Stream<Stream<Component>>`, I can retain the whole chain of functions and get back a simpler `Stream<Component>`.

It is beautiful. I am so glad I did not get lost in all this chaining. This is functional programming and I am back at home now. Functional programming in Dart and Flutter!

What do these lines do?

I combine two streams. One may carry the site id, another may carry the product id.
Whenever these two streams have at least one value, I will start getting events.
For each event, I checked how much information I got. Only site id? Only product id? or Both.
For each combination, I will do different things. The first two will display a redirection, for example.
If I got both ids, then I will start another 2 Streams, asynchronous operations that retrieve the product and site information.
I zip the two Streams and send the event into a Component building function.
Because the latter combo needs to return a Stream for the asynchronous operation, I need to return Streams for all three combos.
If I use `map` instead of `flatMap` in line 3, I would end up with a Stream<Stream<Component>>. Instead, I have a Stream<Component> to work with now.

Monad Chaining and FP. Yay!