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Haskell to JavaScript compiler, based on GHC

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Google Summer of Code is participating in Google Summer of Code again this year. If you're looking for a fun project to work on over the summer and want to learn something about Haskell compilation, a GHCJS-related project might just be the ticket for you. We've compiled a list of ideas to get you started:

GHCJS Google Summer of Code project ideas

Haskell to JavaScript compiler

GHCJS is a Haskell to JavaScript compiler that uses the GHC API. Our first official release is planned to coincide with the release of GHC 7.8.1.

GHCJS supports many modern Haskell features, including:

  • All type system extensions supported by GHC
  • Lightweight preemptive threading with blackholes, MVar, STM, asynchronous exceptions
  • Weak references, CAF deallocation, StableName, StablePtr
  • Unboxed arrays, emulated pointers
  • Integer support through JSBN, 32 and 64 bit signed and unsigned arithmetic (Word64, Int32 etc.)
  • Cabal support, GHCJS has its own package database

And some JavaScript-specific features:

  • new JavaScriptFFI extension, with convenient import pattens, asynchronous FFI and a JSRef FFI type,
  • synchronous and asynchronous threads.



The GHCJS command line options are still in flux. Make sure that you have the
latest Cabal patch installed when you update GHCJS. You need both an updated
`cabal-install` executable and `Cabal` library. Rerun `ghcjs-boot --init` to get the
latest updates for the libraries. If you get `Not in scope: data constructor ‘GHCJS’`,
you need to update your `Cabal` library.

First install GHC 7.8 release candidate 2 or later and check with ghc --version that it's the compiler in your PATH. Next, make sure that you have all the prerequisites for your platform:

Linux / OS X

  • a recent version of alex and happy need to be in your PATH
  • git, make, cpp, autoreconf, patch need to be in your PATH
  • by default, ghcjs-boot will try to use the system GMP library, see ghcjs-boot --help for more info.


  • virus scanners often interfere with configure scripts (permission denied errors), disable on-access scanning before running ghcjs-boot.
  • no extra programs need to be installed, ghcjs-boot will download an archive (around 100MB) with the required programs.

Run the following script to install an updated Cabal and cabal-install with GHCJS support. Note that this will overwrite the cabal executable in your cabal executable installation path (typically ~/.cabal/bin), you might want to backup your current version or append a custom program suffix with a cabal option such as --program-suffix=-js.

git clone
cd cabal
git checkout ghcjs
cabal install ./Cabal ./cabal-install

Make sure that you're now running the new cabal-install, GHCJS support must be listed under the compiler flags:

$ cabal install --help
                                build files (default dist)
 -g --ghc                           compile with GHC
    --ghcjs                         compile with GHCJS
    --nhc98                         compile with NHC

Now install GHCJS itself:

$ git clone
$ cabal install ./ghcjs

Check that you have the correct version of GHCJS in your PATH:

$ ghcjs --version
The Glorious Glasgow Haskell Compilation System for JavaScript, version 0.1.0 (GHC 7.8.1)
$ ghcjs-boot --version
The Glorious Glasgow Haskell Compilation System for JavaScript, version 0.1.0 (GHC 7.8.1)

Build the base libraries for GHCJS:

$ ghcjs-boot --init
$ ghcjs-boot --init --with-cabal cabal-js # if you used --program-suffix=-js earlier


ghcjs can be invoked with the same command line arguments as ghc. The generated programs can be run directly from the shell with Node.js and SpiderMonkey jsshell. for example:

$ ghcjs -o helloWorld helloWorld.hs
$ node helloWorld.jsexe/all.js
Hello world!

Use cabal install --ghcjs packageName to install a package

Most packages from hackage should work out of the box. The main exception is packages with foreign (non-Haskell) dependencies. For these packages a JavaScript implementation of the dependencies must be provided. If a package you want to use does not work, please create a ticket.

Use ghcjs-pkg to manipulate the GHCJS package database

The package database and runtime files from the shims repository are kept in the GHCJS application data directory, typically ~/.ghcjs/. Remove this directory to reset your GHCJS installation, you need to run ghcjs-boot --init again.

See GHCJS introduction for more examples.

Hacking GHCJS

If you want to hack on GHCJS, please join our friendly community on IRC at #ghcjs on freenode (You're also welcome if you only use the compiler or just want to chat about it!). The channel can be quiet from time to time, but with a little patience you should be able to find people to get you started or point you in the right direction.

The repositories you will be interested in are:


    The compiler itself, code generator, linker, test suite


    • Executables ( src-bin )
    • Main code generator ( src/Gen2/Generator.hs )
    • Optimizer ( src/Gen2/Optimizer.hs )
    • Linker ( src/Gen2/Linker.hs )
    • Generated runtime code (h$ap_x_y etc): ( src/Gen2/RtsApply.hs )

    Implementation of much of the runtime system and JavaScript dependencies. Code from this repository is linked into the lib.js and lib1.js files when generating a .jsexe. Installed in ~/.ghcjs/version/shims by ghcjs-boot.


    • thread scheduler, MVar implementation ( src/thread.js )
    • STM implementation (src/stm.js)
    • IO system ( src/io.js ) (buffered IO for putStrLn etc)
    • Heap scanner for weak references and finalizers ( src/gc.js )
    • package-specific dependencies (pkg), dependencies listed in packagename.yaml (temporary measure until Cabal supports JavaScript dependencies directly)
    • third party libraries (lib)

    GHCJS base library, basic marshalling between JavaScript and Haskell data

If you've changed something that affects the code generated by GHCJS, you'll need to rebuild the base packages (reboot the compiler). Often, a full reboot is not required. You can run a quick boot to install only the essential packages (e.g. base, integer-gmp, array, but not ghcjs-base). Adjust the -j4 flag (number of concurrent jobs) for your system:

$ ghcjs-boot --quick -j4

After a change, please run the test suite to check that it does not break anything. Install jsshell and node.js first to make sure that the tests run correctly (js and node executables should be in your PATH)).

When the compiler has been booted, run cabal test, or run the test program directly for more options:

$ ./dist/build/test/test --help

For example to run a specific test or tests that match a pattern:

$ ./dist/build/test/test -t pattern

New test cases, in particular if you add new features, are always welcome. The testrunner automatically picks up new tests (.hs files that start with a lowercase letter) in the existing categories.

Debugging GHCJS programs

TODO add better instructions here

  • link your program with -debug: ghcjs -debug -o test test.hs, this adds debugging information to the generated code
  • get more informaton from the runtime system by enabling various tracing and logging options. See the code in the shims repository for more info. Examples:
    • -DGHCJS_TRACE_SCHEDULER: messages from the thread scheduler
    • -DGHCJS_TRACE_CALLS: print all function calls from the main loop. warning: lots of output. Requires -debug for name information
    • -DGHCJS_TRACE_STACK: print top of stack for every call in the main loop. warning: even more output
    • -DGHCJS_TRACE_WEAK: output related to weak references
    • -DGHCJS_TRACE_STM: output for software transactional memory
    • -DGHCJS_TRACE_GC: output garbage collector (heap scanner) related messages
  • see the utility programs in utils in the ghcjs repository for tools for inspecting object files, quickly bisecting bugs in the optimizer etc.

Profiling GHCJS programs

You can use the JavaScript profiler to get a basic idea where the majority of time in your program is spent. Non-exported names (like stack frame functions) start with h$$, scroll up in the code to find an exported name to which they belong.

Unfortunately due to Haskell's lazy evaluation and GHCJS's tail-call optimization, the information here is far less useful than GHC profiles for native code. Profiling support is planned, but implementation has not been started yet. Profiling will support cost centres like in native GHC. In particular, we intend to add memory profiling support for interactive (reactive) systems, keeping track of allocations and retention per event. If you're interested in helping out or discussing features, please contact us in #ghcjs on freenode.

JSC and webkit

Applications that use the following libraries should compile with GHCJS and run in a modern web browser and interface with DOM and JavaScript in the browser.

  • webkit - Bindings for WebKitGTK+ that provide a low level DOM interface.
  • webkit-javascriptcore - Low level bindings for JavaScriptCore
  • jsc - Higher level inteface for JavaScriptCore

You can use these libraries without GHCJS to build a native version of your application (it will use WebKitGTK+ to run without a browser). If you want to find out more about making GHCJS compatible Haskell applications check out the GHCJS Examples

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