Application-as-a-Function Thinking

In a stateful system (where outputs depend on previous inputs) you need to have access to all previous inputs in order to determine the output of some input, right? Also keep in mind that this is a specification, not an efficient implementation. If you'd implement it like this you'd have to recompute the output based on the inputs over and over again for each new input. And yeah, you can think of it as some kind of functional composition as nine_k pointed out.

> This sounds very similar to DDD (like as described as the DDD book at the end of the blog post).

Yeah, I think DDD gets this right: some lightweight methods (e.g. event storming) around inputs and outputs. But yeah, Cleanroom did this in the 80s.

> At IBM direct, or are there any particular resources you can point to which you found useful?

On this particular topic, see Harlan Mills' "Stepwise Refinement and Verification in Box-Structured Systems" (1988): https://trace.tennessee.edu/utk_harlan/16/

> ... as a thought experiment it makes sense but it's not transferrable to the real world.

For the record, it's certainly not a thought experiment, I have been using this in the real world for a few years.

> Like take the log out action as an example...

The problem you outline with regards to intermediate UI states around the logout flow example is one of assumed responsibilities. If an application needs to show some specific transient UI when the application has moved between two states well, that's a UI concern and does not have to be modelled as part of the core application state. For example if you have a UI for an application which is required to show some jazzy animation when a user transitions from logged-in to logged-out that in no way needs to be part of the core application logic. The UI can just inspect the incoming state and conditionally when detecting a move from LoggedIn to LoggedOut trigger some UI animation / transient dialog / whatever.

One nice thought experiment / thought pump I find of value when dividing core application (~domain) and UI responsibilities is thinking about if you were to switch the UI from say a mobile framework (say Android/Compose) to a desktop terminal, what stuff stays the same and what stuff is display specific. In the above example you most likely would not want to show a jazzy animation on the terminal and therefore is display specific and should no be modelled in the core application logic. This is a nice simple thought process for dividing these responsibilities.

> And then how do we deal with stuff that's actually state. Like a database or whatever. Are we really passing that as a function parameter so that 10 calls deep can use it?

No, a database would fall under IO and would make more sense to access behind a Command interface in this pattern, like most other side-effect interactions. This fits into the "functional core imperative shell" mindset referred to in the post.

> And what happens 6 months in when we want a caching layer? Do we go back and edit every single function to accept a Redis argument now? Realistically...

I am in no way experienced with BE caching layers, my experience is mostly client side mobile applications. However, my first approach for an in-memory DB cache would be to chuck that in front of the DB access behind the same abstraction, and also this would be behind the Command interface, again like all other side-effect interactions.

> For me the difficult part was where the side effects fit in and how they're processed, what you call "commands" and "command handler"

Do you mean difficult for you when designing, or difficult to understand in my post?

In my experience the commonality found between client side mobile applications and reactive principles is essentially everything is a stream, and the UI subscribes to those streams.

The approach outlined in the post is similar in terms of the UI observes a (single) stream of data, which can just be a simple (State) -> Unit) as opposed to some reactive library stream primitive.

The approach outlines is different from all the "reactive" client side applications I have seen as there is no proliferation of stream primitives present throughout the codebase, which often seems to create complex code with very little if any user benefit.

I'm not sure what other elements you see in the post which also fall into the "reactive paradigm"?

I am assuming when a user interacts with one control button / dial / interaction it has a cascade effect on other controls / displays etc?

I am also assuming you already need to have a data structure representing the complete state of the system no?

There are multiple interesting aspects here, like are you / do you need to utilise complex state machines here? Just the subject of imperative vs functional state-machines is interesting and a small part of the wider approach.

Another interesting area is it depends on the language and execution environment you are operating in and how expensive operations over immutable data structures are.

Lots of questions!