[troym]

> Once you scratch a little bit the surface, you will see the extraordinary power of its persistent data structures.

I've heard this repeatedly, yet in the examples I've read (for web servers and for web ui work with cljs), they're using atoms with `swap!` to provide "current state".

Could you provide an example of using persistent data structures in a web app? Or am I mis-understanding?

[danneu]

You're compare-and-swapping out persistent values with an atom (it holds a succession of immutable values over time). It's not an example of people shirking persistent data structures for mutation.

[troym]

Yes, I understand that. I guess my original question would have been better put: can the notion of "current state" be avoided with persistent data structures, and if so, how?

[erikpukinskis]

I'm a hardcore imperativist right now, so don't take this as authoritative, but here's my understanding of how that works:

Think of your data as one reality in the set of all possible realities. None of them exist or don't exist, they'll just receive human attention or they won't. Instead of an inbox with an ID and a current state, you just store events that happened and a user might say show me the end state of the timeline that started at event d79se5k. They might also ask you for the timeline starting at a different point.

There's no "current state". You're just ready to answer questions about hypothetical realities. And while some realities are more likely to be computed (and cached) that's only because they correspond to some key facts that a human is going to look up.

Put another way, imagine a soup of unrelated facts. Your code is a way to project those facts into different mathematic spaces. You never just say "what's the current state" you have to actually write out specifically what you mean as a query.

Sorry for the vagueness. I'm just putting out there my fuzzy understanding until someone else can write something better. :)

I'm really interested this approach, and I think a lot of data should be immutable. Sadly, a lot of people are treating immutability as a religion these days, which causes problems when computation is better suited to imperative structures.