H2JVM – A Haskell Library for Writing JVM Bytecode

Posted by rowbin 2 days ago

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Comments

Comment by J-Kuhn 17 hours ago

At a quick glance, it seems to be missing some hook to resolve class hierarchies - which is needed when merging stacks with different types. Consider

    var foo = expr ? new Foo() : new Bar();
For such an expression, some superclass or superinterface for both Foo and Bar has to be chosen.

What I am curios about is, if the java classfile API[1] provides a good model that could work in Haskell as well - I always had the impression that it was heavily influenced by functional programming, for example it uses "lifting transforms"[2].

PS: Good work on the label resolving part - this model is also used by the Java Classfile API and before it, ASM.

[1]: https://docs.oracle.com/en/java/javase/26/docs/api/java.base... [2]: https://docs.oracle.com/en/java/javase/26/docs/api/java.base...

Comment by amarant 17 hours ago

This... Confuses me profoundly. My entire career I've worked with Java, and it's mostly a pretty decent language, imo. I think my biggest gripe with Java is the JVM. It's limiting, doesn't really provide any value(the proposed value is portability, but we always run apps in docker containers anyway, so what is it really doing for us?)

I (kinda) get why someone might want to write Haskell rather than Java, but I'm just not sure why you would want to run Haskell on the JVM?

Comment by cogman10 15 hours ago

Wow, I think I have almost the exact opposite opinion here.

Java is an ok language, but what really makes it shine is the JVM. It's one of the fastest VMs out there and is one of the most customizable ones as well. For example, pretty much all other languages with a GC have just a GC and that's it. Java allows you to pick and choose your GC based on the workload.

It is one of the least limiting VMs out there because any knob you might want to tune, can be tuned. It's a huge value add.

I think the only part of the JVM that's not great is the fact that objects are bulky and the lack of value classes. Which ultimately means every struct like object you want can have a pretty hefty price in terms of memory. But otherwise, it's best in class basically for everything.

Comment by tikhonj 17 hours ago

This project isn't for running Haskell on the JVM, it's for writing a compiler that produces JVM bytecode. You'd use it if you wanted to implement your own JVM language in Haskell or maybe if you wanted to have some kind of JVM-backed domain-specific language embedded in Haskell.

Comment by pron 17 hours ago

The reason most "serious" or important software is written for the JVM these days is because it gives you an unparalleled combination of performance, productivity, and observability. There's almost no competition if these things are what you need. The problem isn't so much why pick Java among the alternatives, but that there are hardly any alternatives.

Comment by AlotOfReading 14 hours ago

You have to put on some very narrow lenses to argue that "most" serious software is written for the JVM. Operating systems, compilers, browsers, databases, planes, cars, the transaction processing for at least one major payment processor, major cloud services, etc predominantly use other languages.

I don't think there's any single language that most serious software is written in.

Comment by pron 14 hours ago

C, C++, and C# are obviously also major players in "serious software", but you can estimate the volume of software through the number of people involved (e.g. https://www.devjobsscanner.com/blog/top-8-most-demanded-prog...). If Java doesn't have an outright majority, it has an obvious plurality. And again, there aren't many alternatives.

Comment by vips7L 17 hours ago

An interesting take.

You get lots of things for free when targeting JVM bytecode. GCs, JITs, interop with one of the largest and most battle tested ecosystems.

Comment by rienbdj 17 hours ago

Pretty good performance for low effort is a big win.

Comment by 17 hours ago

Comment by internet_points 17 hours ago

see also https://github.com/Frege/frege and https://github.com/mchav/froid (though both are kind of dead I guess?)

Comment by J-Kuhn 17 hours ago

There is also Idris 2 for JVM (https://github.com/mmhelloworld/idris-jvm)

Frege targets Java source code, which is then compiled by javac - the downside of that approach is you can not preserve the line numbers for debug information.