Structured Concurrency Definition (2019)
gavinhoward.com
gavinhoward.com
First, instead of calling a function, just add that function to a list of functions. Then when you're done, spawn a thread and pass it the needed data and the list of functions. The thread will work its way through the chain.
This technique is possible because "closures are just a poor man's object." (I think I saw that on the c2 wiki.)
Yes; the inverse is also true :) The "koan" is on c2 here: http://wiki.c2.com/?ClosuresAndObjectsAreEquivalent (with a link to its original source).
One thing I don’t think I’ve seen mentioned on those pages is the distinction between a “closed” object you can only send messages to and an “open” one you can inherit from / delegate to / mix in / COM-aggregate: the former is indeed a closure, that is some code and an environment, that you feed the message selector and some arguments; the latter is also some code and an environment, but you feed it the resend target, the message selector, and some arguments. So depending on what exactly you’re calling an object, an object is a closure... in one of two non-equivalent ways. Maybe not really a closure then?
See also for not-quite-identical points of view: Piumarta’s note on prototypes in his COLA system[1]; Chisnall’s description of the (IIUC now-dead) new Objective-C runtime for Étoilé[2].
The point is more that there is not a set of "closure problems" and a set of "object problems" that can only be solved with the relevant technologies. You can always put together a solution to a problem with the other technology. It may have different cost/benefit tradeoffs. It may have different affordances. I would certainly advise against walking into an object system and trying to force it to solve every problem by objects functioning as closures, and I would advise against walking into a closure system and trying to force it to solve every problem by forcing closures to work like objects. One should work with the grain of the wood you are working in rather than insisting on treating the wood like the pottery you are more familiar with.
But it is always helpful to remember that within every object system is a closure system if you really need it, and within every closure system is an object system if you really need it. The resolution to "how can each be a subset of the other without them being equal to each other" is that there is some equivocation here. The object system that closures can implement conveniently is less rich than a dedicated object system, and vice versa. But if you just need to momentarily borrow the alternate concept, you probably don't need all the frippery you need if it is your primary tool for interacting with the world, so it works out.
If you found this interesting then you shall like the following pages:
The Vale language blog post on seamless structured concurrency https://verdagon.dev/blog/seamless-fearless-structured-concu...
The rust book on concurrency https://doc.rust-lang.org/book/ch16-00-concurrency.html
I've been working on implementing Java async/await state machine with switch statements and a scheduling loop. If the user doesn't await the async task handle, then the task's returnvalue is never handled. This is similar to the Go problem with the go statement.
https://github.com/samsquire/multiversion-concurrency-contro...
If your async call returns a handle and the handle is linked to a scope then in theory the async/await could be used as primitive to structured concurrency.
I journal about asynchrony and parallelism everyday in ideas4 (see my HN profile)
It's interesting to see this posted after three years.
In fact, there should be "(2019)" in the Hacker News title.
After looking at the definition, I think it holds up well. I would probably tweak it, but I was three years less experienced back then.
It's weird to think that I posted that just two months before lockdown. It seems like I posted it long before that...
It refers to Trio nurseries as “scopes”. I thought about them like that for a while, but don’t think that’s not exactly the right view now. As a subroutine, you can’t (usually) reach up into your parent static or dynamic scopes and cause new objects to be created there and outlive your own dynamic extent. (Tcl exempted.)
Because nurseries are first-class objects, though, you’re free to create a root nursery at startup and stick it in a global variable, after which everyone is free to create as many Go-style unaccountable threads as they want to. It’s just that you don’t have to. Yet passing worker threads their originating nursery so that they can spawn more peer workers is a moderately useful pattern.
TL;DR: Firsr-class nurseries are not quite scopes because they don’t enforce strict lifetime nesting between a spawned thread and the thread that caused it to be spawned.
When I read the Go statement considered harmful post, I felt the same thing. This just makes sense, everything should be like this. A number of these ideas have been designed into Kotlin’s concurrency library, what are your thoughts about it?
I haven't seen Kotlin's library, so I can't comment.
Do you have a link? I wasn't able to pull up anything good with a search.
To answer the GGP, this looks like true structured concurrency to me. Chalk one up for Kotlin!
Not really. I've had people go after me for such things.
Definitely a good thing. But does that really justify the label "structured concurrency"? I was rather hoping to see something more, especially since languages like Rust, Go, JavaScript (and even Python) go much further.
The entire point of structured concurrency or structured programming is to build limitations into the system, then build other guarantees on those limitations. If you don't have the limitations, you don't have the things you can build on it. Systems lacking those limitations do not "go beyond" the limitations, they lack the ability to build the guarantees.
I have been influenced in my Go programming by this essay. A lot of my concurrency is actually "structured concurrency" if you look at it. But you have to look at it to see, because there is no way for me to guarantee it structurally. No matter how much I use the ideas, no matter how much I were to write it into an API, I can never prevent anything from blasting a "go" statement whereever they want, whenever they want, and violating the limitations, and thus breaking the guarantees. This is not the end of the world some programmers treat it as; no language can do everything and enforce everything and be perfect without the programmer having to bring some discipline to the party. But neither is it no big deal at all and something you can just wave away with a vague statement about how some programmers just need to be better programmers or something. These things matter.