I'm far from an expert on this topic, but it seems that you're still missing the main point--this talk is exactly about how to model time dependent data (immutable data), and how not to (mutable state, oo). Hickey definitely isn't advocating a system that can't change with time. Such a system would be pointless. He wants changes in state (which, naturally, occur on a time axis) to be represented by new values, not as in place, destructive updates of old values, as it's done in oo and currently popular databases.
If you look at the results of this approach in Datomic, I think you actually do see a design that treats time as much like a first-class citizen as it's ever been treated, in the sense that time essentially acts as a primary key, and developers are provided with a time machine that allows them easy and efficient access to any state that has existed in their data in the history of the application. (In theory, at least--I haven't personally tried Datomic).
I’m pretty sure I understand where Rich is coming from. I’m just arguing that while moving to persistent, immutable values is a big step in what could be a good direction, it’s not sufficient by itself to justify or cause a shift in mainstream programming styles on the scale of abandoning OOP (as suggested in the original post I replied to).
You lose things in that transition, very useful things that are widely applicable. We’re not going to just give those up without having something good enough to replace them, and I thought that in this specific talk those two areas I mentioned were glossed over far too readily.
For example, although Rich said very clearly that he thought it was OK to manipulate data in-place as an implementation detail of how a new value is built, he then argued that once the finished result was ready it should become an immutable value, and that we no longer need to use abstractions that are based on or tied to that kind of underlying behaviour. I contend that there are many cases where it is not so simple even with today’s technology, and that the idea of constraining in-place mutation to the initial creation of a value is a leaky abstraction that will not survive a lot of practical applications.
Later on, processes are briefly mentioned, but that part of the talk is about information systems, which are mostly concerned with pure data analysis. That makes it is rather easy to dismiss the idea of modelling interactive processes in context, but unfortunately, very many real world programs do need to be concerned with the wider time-related concepts like effects.
I’m sure Rich himself is well aware of all of these issues. He’s discussed related ideas in far more detail on other occasions, including in the talk that lrenn cited above. But I find his challenge near the end of this talk, “If you can afford to do this, why would you do anything else? What’s a really good reason for doing something else?” to be rather unconvincing. For one thing, that’s a mighty big “if”, whether you interpret “afford” in terms of performance or dollars. For another thing, the answer to those questions could simply be “Because ‘this’ isn’t capable of modelling my real world, interactive system effectively.”
If you look at the results of this approach in Datomic, I think you actually do see a design that treats time as much like a first-class citizen as it's ever been treated, in the sense that time essentially acts as a primary key, and developers are provided with a time machine that allows them easy and efficient access to any state that has existed in their data in the history of the application. (In theory, at least--I haven't personally tried Datomic).