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Java Code Generator for Idris
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Idris Java Backend

This is an updated version of the Java backend for Idris.


  • Java Arrays
  • Port more of the Iris libraries that relie on IO (e.g. IdrisJava.System needs finishing)
  • Read an attribute/field
  • Write an attribute/field
  • Class definitions
  • Support anonymous classes
  • Allow multiple methods in an anonymous classes
  • Write basic tutorial
  • Detailed documentation
  • Setup testing framework


First and formost we are jargeting Java and the JVM and an update version of the Java runtime and JDK must be installed, they can be downloaded here:

Make sure you have a recent version of GHC installed, this has all been tested with 7.10.2. If you are new to GHC, then I recomend using the Haskell Platform.

You need an update to date version of Idris, at least 0.9.19 but I would really get it from clone from the dev branch.

Its been tested with an updated version of the Idris Java runtime and Idris development version 0.9.19.

One thing to note is that as the Maven Repository has a version of the Idris Java Runtime that is compatable with 0.9.14, to use this version of the frontend you will need to install a built version into a local repository. This can be done with the following commands:

git clone
cd idris-java-rts
mvn compile
mvn package
mvn install:install-file -Dfile=${project.basedir}/target/org.idris-lang-idris.jar

Then you can build the code generator in a sandbox with Idris dev. For instance if you have a directory you can create a toplevel sandbox as follows:

cabal sandbox init --init .

then clone idris-dev with:

git clone
cd Idris-dev
cabal sandbox init --init <idris-root>

then follow the rest of the instructions on the Idris wiki to build the compile. Note: please be sure to to build with FFI enabled.

Add the /bin to your path (e.g. export PATH=...). Now your ready to build the Java code generator. Clone and bujild idris-java with the following commands:

git clone
cd idris-java
cabal sandbox init --init <idris-root>
cabal install --only-dependencies
cabal configure
cabal install

Assuming all went well, then you will have the executables idris and idris-java (along with other backends) in the directory /bin.

Currently there is an absoutely bare bones Java specfic runtime, which can be installed as an Idris package. This can be found in the directory libs. To install the package, simply run the following commands from the lib directory:

idris --build idrisjava.ipkg
idris --install idrisjava.ipkg


In general, there is no support for the default C IO monad and so while it will work if you only call putStrLn, for example, as it will not work in the general case. Instead you should import the JAVA_IO monad found in the module IdrisJava, in the package idrisjava. To compile with the Java backend, using the idrisjava package use the command line:

idris --codegen java -p idrisjava file.idr -o a.out

where file.idr is the Idris file to compile and a.out is the resulting executable program.

Here is the standard hello world example (hello.idr), but for fun using a foreign call out to Java to handle the printing.

module Main

import IdrisJava

systemoutprintln : String -> JAVA_IO ()
systemoutprintln s
   = invoke "System.out.println" (String -> JAVA_IO ()) s

main : JAVA_IO ()
main = systemoutprintln "Hello, World!"

which when compiled with:

idris --codegen java -p idrisjava hello.idr -o a.out

produces the output:

Hello, World!

Current status

The update is in progress and so far I have tested it with basic programs and the most resent updates support calling Java from Idris code, using a JAVA_IO instance of IO'.

Currently there is no support for allocating Java classes, on the list, and it is also not possible (currently) to call Idris code from Java.


The original work on this backend happened within the Idris compiler tree, but at some point (2014) Edwin Brady forked it out into Idris Hackers Git repo, where it seems to have gone rotten. I picked it up to compare against some other work I'm doing with the Idris compiler and I simply needed a working version!

Here are Edwin's original commit comments:

"This is extracted from the Java backend in Idris-dev. It is untested! Please feel free to test, maintain, etc :)."


Drop me a line if you have any questions bendict.gaster at

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