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0 pointsChris_Newton13y ago0 comments

For the avoidance of doubt, when I’m talking about (not) doing away with a time dimension here, I mean from the perspective of the world changing over time as our program runs, not of associating values that effectively last forever with a certain point in time (as in purely functional data structures, version control systems, etc.).

That is, even if we follow the current trend of adopting more ideas from functional programming in mainstream programming languages, I’m saying that I doubt we will ever completely remove variable state, which is what I understand Rich to mean by “place-oriented programming”, or events that must happen in a certain order.

Instead, I think we will learn to control these aspects of our programs better. When we model time-dependent things, we want to have well-specified behaviour based on a clean underlying model, so we can easily understand what our code will do. Today, we have functions and variables, and we have type systems that can stop us passing the colour orange into a function eat(food). Tomorrow, I think we’ll promote some of these time-related ideas to first-class entities in our programming languages too, and we’ll have rules to stop you doing time-dependent things without specifying valid relationships to other time-dependent things. Some of the ideas in that second talk you linked to, like recognising that we’re often modelling a process, are very much what I’m talking about here.

As an aside, it’s possible that instead of adding first-class entities for things like effects, we will instead develop some really flexible first-class concepts that let us implement effects as just another type of second-class citizen. However, given the experience to date with monads in Haskell and with Lisps in general, I’m doubtful that anything short of first-class language support is going to cut it for a mainstream audience. It seems that for new programming styles to achieve mainstream acceptance, some concepts have to be special.

In any case, my hope is that if we make time-related ideas explicit when we care about them, it will mean that when we don’t need to keep track of time, we needn’t clutter our designs/code with unnecessary details. That contrasts with typical imperative programming today, where you’re always effectively specifying things about timing and order of execution whether you actually care about them or not, but when it comes to things like concurrency and resource management the underlying models of how things interact usually aren’t very powerful and allow many classes of timing/synchronisation bug to get into production.

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danenania13y ago

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).

Chris_NewtonOP13y ago

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.”

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danenania13y ago

Chris_NewtonOP13y ago

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