You may want to have a look at my little HJVM project on Github ( https://github.com/JPMoresmau/HJVM). Promise, I'll put in on Hackage some day. Basically it provides FFI wrappers and some c code to be able to start a JVM and call Java methods, etc from Haskell. -- JP Moresmau http://jpmoresmau.blogspot.com/ On Tue, Nov 20, 2012 at 7:53 AM, Mathijs Kwik wrote:

KC writes:

...

Instead of Haskell running on the JVM is there a way for Haskell to call a JVM language (or generate bytecode) to access the Java class libraries when needed?

I once did a small test to get this working. It's not that hard, but needs some work. It's fine for exposing a few functions though.

Basically it's a 2-step process, eased by using a makefile or similar helper.

You have to compile your haskell code into a shared object (.so on linux, .dll on windows), which includes the haskell runtime (rts).

This library can be called from c. A small pitfall is that you first need to do a call to initialize the haskell runtime, and when you're done using it, close it. This is most easily just tied to your c/java program's main initialization functions.

Java is able to load/use these shared libraries through JNI. Of course you lose your platform-independance, so if you want your java application to work on multiple platforms / OSses, you need to build shared objects for all of them.

Last but not least: You have to export the haskell functions you want through FFI. Also, make sure they use raw data types such as CString, as that what C and java will give you and expect back.

So basically you go Haskell FFI <-> C <-> Java JNI

I'm sorry I cannot give you any links or code, because I'm in a bit of a hurry. But google and the ghc docs are your friend.

Mathijs

...

Or

Is there a way for a JVM language or bytecode to call Haskell when

needed?

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On Tue, Nov 20, 2012 at 11:01 AM, Ilya Portnov wrote:

Please take a look at http://hackage.haskell.org/package/hs-java also. It's an assembler/disassembler of Java bytecode and *.class files. Moreover, there is https://github.com/MateVM/MateVM — an (experimental) Java VM on Haskell :) So, if you are interested in haskell/java interop, maybe we could integrate our efforts.

To elaborate the current state of MateVM a bit: We implement methods declared as native in Java by either IO Haskell functions or C functions. You can hook a Haskell function here in the source [1]. There're already some examples. Drawbacks are: (1) you have to mess around with the internal object layout if you want to pass values, which is JVM specific (2) it is *not* the standard way, i.e. it isn't like in JNI defined (3) we'll probably change the interface sooner or later ;) If you want to play around, you're welcome. But if you need something stable, don't use MateVM at the moment :-) it's like two man-years away from something productive usable. Bernhard [1] https://github.com/MateVM/MateVM/blob/master/Compiler/Mate/Runtime/MethodPoo...