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Last pushed: 2 years ago
Short Description
A build of my fork of idris-java
Full Description

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
  • [x] 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
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 <idris-root> 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 <idris-root>/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 <idris-root>/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


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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