Comment by jillesvangurp | Hacker News Reader

Comment by jillesvangurp | Hacker News Reader

Parent Full thread

It's more than that. Java developers are switching to Kotlin; notably on Android and Spring code bases (well over 50% of backend Java projects at this point apparently).

This is an article about structured concurrency and how Loom implements something that Kotlin has shipped that both Android and Spring heavily integrate with already.

So, there's more than a casual relation between Kotlin co-routines and Loom. I'd go as far that they apparently took a really good look at it and somehow ended up with something that essentially implements almost 1 to 1 the same kind of things.

Roman Elizarov (tech lead for co-routines, and recently the whole language) has been talking about structured concurrency a lot. I don't think he invented the notion but it definitely is what co-routines is designed to do.

So, when I see an article talking about how great Loom and structured concurrency is mentioning several languages but yet somehow glossing over Kotlin, I call it out as lame. Oracle no doubt has good reasons to not want to talk about Kotlin. But to me it is clear they consider it a threat. It's not the first time that they celebrate a few features new to Java where they mention other languages as influence and yet not mention Kotlin. Recent introduction of Records is a good example.

The Threading APIs date back to the late nineties and are full of complex stuff that you need to be aware of if you go near them if you care about avoiding all sorts of interesting categories of bugs, issues, and pitfalls. So much that I'd treat the occurrence of import java.lang.Thread as a giant red flag in a code review. Typically, it's a rookie mistake to use that. You shouldn't have to; there are better APIs.

Kotlin, btw. treats threads just as a special case of co-routines. You can have co-routine dispatchers that represent a particular executor. This is how you separate slow blocking IO things from e.g. CPU heavy stuff, or UI event handling. So, there is no API split in Kotlin. Some co-routines use a threadpool, some don't.

> notably on Android and Spring code bases

Of course they are, on Android, Google is doing everything that they can to push Kotlin down developer throats, most Java developers end up moving to Kotlin when the option is an half baked implementation of Java, called Android Java, stuck in Java 8, when we are already on Java 15, and Google is unwilling to move Java support any further than what they are forced to due to InteliJ own changes.

Spring, well they only care about marketing and getting Spring sales, I still remember when Groovy was supposed to take over the Java world and also had first class support in Spring, just like they are selling Kotlin nowadays.

I got some nice consulting gigs porting those projects back to Java, and the same will happen with Kotlin projects when Google gets fed up with Android and moves on to Fuchsia/Flutter.

> So, there's more than a casual relation between Kotlin co-routines and Loom.

Not beyond the fact it was one of the things we looked at and decided to go in a completely different direction. We looked at Python, Go, JavaScript, C#, Kotlin, C++, Rust, Zig, Haskell, Pony, Scheme, Erlang, Céu, Scala and Clojure, and decided not to go down the C#/Kotlin path, which is why we ended up with a solution that is nothing alike. The positive influences were Erlang, Go, and Scheme (with a glance to Céu). That's why virtual threads share a lot of similarities with Erlang processes and Go goroutines, and borrow ideas from Scheme's (and OCaml's) multi-prompt delimited continuations, but are not at all like C#/Kotlin's syntactic coroutines.

There are certainly things about Kotlin we love, like nullability types, and that we'd like to see in the Java language some day. When we do, Kotlin would have been the influence. Syntactic coroutines, on the other hand, were something to avoid.

> Java developers are switching to Kotlin

Just as they had to Scala in the past, many developers are switching to Kotlin, which has reached ~2-4% on the Java Platform, and might even reach 5-7% some day. That is huge, perhaps unprecedented, market penetration for a Java Platform language, but let's not get carried away.

There are certainly languages we consider serious competitors, but Kotlin is still an order of magnitude in size away from being one of them. Java is so big that you can still be 20-50x smaller and still be a very popular language, yet not quite a competitor.

> Recent introduction of Records is a good example.

Nope. First of all, Kotlin doesn't have records. Kotlin-like data classes were something we looked at (we look at all languages) and said, we don't want that, we want records. Second, the inspiration for records was ML. So records are actually yet another example where we decided not to go in the same direction as Kotlin. This isn't to say Kotlin did something worse or better, but it did do something decidedly different.

> Typically, it's a rookie mistake to use that. You shouldn't have to; there are better APIs.

The same APIs Java users use today (Executors) are the ones they'll use when Loom lands. There is still no need to use the Thread API directly.

> Kotlin, btw. treats threads just as a special case of co-routines.

Perhaps in your desire to see Kotlin influences everywhere (and, BTW, syntactic stackless coroutines were done in C# first, at least among the well-known language) you misunderstand what Loom is. Virtual threads are threads, period.

Anyway, it's perfectly fine to prefer Kotlin over Java (the language) just as it is to have the opposite preference. But I think you misjudge the influence those languages have and draw from.

> Syntactic coroutines, on the other hand, were something to avoid.

Could you articulate or point me to some of the arguments or thoughts that lead to the conclusion here? I know and understand the term "colored functions", but I'd love to read a real analysis of the pros and cons of colored functions, because I personally go back and forth on whether I think they are bad or good. On the one hand, having a function that is explicitly marked as "this thing needs to be treated specially because it may block for a long time" is actually kind of nice. On the other hand, it's hard to write generic functions/tools when you have to handle different colored closures, for example. Also on that hand is the fact that I can still write a function as the "wrong color" if I'm inept.

Or was that decision based on something other than language semantics?

> There are certainly languages we consider serious competitors, but Kotlin is still an order of magnitude in size away from being one of them.

Just out of curiosity, what are those languages? C#? PHP?

Also, this statement makes you sound like Goliath. I don't disagree with you that Kotlin, in particular, is probably not a huge "threat" to Java. But you're slinging numbers like a politician or a PR person: "Kotlin, which has reached ~2-4% on the Java Platform, and might even reach 5-7% some day". Java has 25 years of legacy- are you seriously saying that picking up 5% of JVM code in < 10 years from the HOST LANGUAGE OF THE PLATFORM is not a little unnerving? Also, do you only care about the JVM? What if everyone switch from Java to C#? Would you still brag that "90% of code on the JVM is Java" even if 0 new projects started choosing Java? I mean, those are unlikely events, but I'm just saying "there's lies, damn lies, and statistics".

The truth is that until pretty recently, Java was really lagging behind and many devs were cursing their fate that they were still working on Java projects. Java 8 was a huge leap forward, and Java 15 is another sizeable leap. So I suspect that Java will stop the bleeding. But it was not at all guaranteed to stay a behemoth, IMO.

> Nope. First of all, Kotlin doesn't have records. Kotlin-like data classes were something we looked at (we look at all languages) and said, we don't want that, we want records. Second, the inspiration for records was ML. So records are actually yet another example where we decided not to go in the same direction as Kotlin. This isn't to say Kotlin did something worse or better, but it did do something decidedly different.

I haven't used them yet, but I think that records look better than Kotlin's data classes. But the only difference I see is that they don't auto-generate a `copy()` method, like Kotlin's data classes do. I believe that was a big mistake on Kotlin's part. Is there some other way that Java records are different than data classes? Because if that's the only difference, it sounds really disingenuous to suggest that records are not inspired by data classes. Like, what are the odds that it took until 201x for you (all) to decide to copy records from ML? You certainly didn't do it before Kotlin for some reason.

> Could you articulate or point me to some of the arguments or thoughts that lead to the conclusion here?

I gave a talk about exactly that recently at Code Mesh. I expect them to post the video soon. https://codesync.global/speaker/ron-pressler/#745why-user-mo...

> Also, this statement makes you sound like Goliath.

Maybe, but it's not just a matter of size but also trajectory. And, as you say, our competition is mostly off the Java platform. My point wasn't to brag, but to put matters in perspective. There are dozens of nascent languages, the vast majority of which will never make it to the top ten, let alone the top five. And while a few of them will, no doubt, one day unseat the incumbents, we obviously don't think of all of them as "threats."

> what are those languages?

The obvious ones. It's not that we don't know that some smaller languages will one day become big, some will even surprise us all, but extrapolating early growth to long-term success is certainly not a good model.

> Like, what are the odds that it took until 201x for you (all) to decide to copy records from ML? You certainly didn't do it before Kotlin for some reason.

As I wrote in another comment, Kotlin's arrival and Java's "boost" are both a response to the same event: Java's decline in Sun's dying days. Once Oracle increased investment, it was mostly a matter of prioritising which features to do first. Records were seen as less urgent as lambdas, so they came later.

You didn't answer a few of the interspersed questions, so I'll press you on the records one.

What about Java records are different than Kotlin's data classes besides forgoing the auto-generated `copy()` method? I understand there are some implementation details, such as inheriting from a Record base class, and how it handles serialization. But I mean as a user.

Can records implement interfaces? Can records be variants in a sealed class hierarchy? Can records have a private primary constructor? Can I customize the getters (to e.g., make defensive copies)?

For other people reading, the answers for Kotlin data classes are: Yes, Yes, Not Really, No.

As an aside, I understand you are an Oracle employee. When you post under this username are you acting in any kind of official capacity for Oracle? Like, is part of your job, so to speak, to have a social media presence? I'm not suggesting anything negative- it's totally fair to invest in outreach, to answer questions, clarify things, etc. I was just curious if I'm talking to "a guy who loves the project he works on" or "a representative of a company".

> What about Java records are different than Kotlin's data classes

Java records, like enums (another feature that is philosophically very similar to records) aim not to reduce the boilerplate of certain operations, but to designate a subset of classes with particular semantic properties (and make those easy to express). In the case of enums, that subset is classes with a well-known, fixed set of instances; for records that is nominal tuples, i.e. immutable, unencapsulated data aggregates, similar to ML's product types. So users, the compilers, and libraries can make certain assumptions about records. For example, their semantic properties (that they are no more than a product of their component types) allow a much better serialization story for them and, indeed, record classes are serialized differently from non-record classes: Instead of invoking a no-arg constructor, their canonical constructor is invoked on deserialization. Their immutability also makes automatic implementations of equality, deconstruction and pattern matching clear and correct.

Just like Kotlin couldn't do user-mode threads efficiently because they have no control over the platform, there was also little point in doing records, because that language's goal was to make it easier, syntactically, to work with the existing Java ecosystem, and, prior to records, Java programmers worked with JavaBean-like classes, so Kotlin tried to make those operations more syntactically pleasant. Java's designers, however, can have an impact on what that ecosystem does and can change its direction.

> As an aside, I understand you are an Oracle employee.

Yes. I work on OpenJDK.

> When you post under this username are you acting in any kind of official capacity for Oracle?

Absolutely not. I speak only for myself. I'm the technical lead for Project Loom, and I want to see what kind of reactions people have to it on social media (as well as conferences, customer meetings, surveys etc.). I guess I see it indirectly as part of my job, at least as far as Loom goes, as these interactions help inform how we explain the capabilities, what features people want etc.. It's nothing official, though. It's also a harmful personal addiction.

> "a guy who loves the project he works on"

Yes, he is Ron Pressler, project lead of Loom - https://twitter.com/pressron

> Can records implement interfaces?

Yes.

> Can records be variants in a sealed class hierarchy?

Yes.

> Can records have a private primary constructor?

The accessibility of the constructor is roughly at least that of the record class itself. If the record class is public, then the canonical constructor must be public. However, thanks to the positive answers to the two previous questions, you can have a readable-though-not-publicly-constructible record -- make it a private implementation of a sealed public interface.

> Can I customize the getters (to e.g., make defensive copies)?

Yes. Although in most cases, defensive copying in the constructor is sufficient.

Reply on news.ycombinator.com