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ghcjs / ghcjs


Haskell to JavaScript compiler, based on GHC


Quick Start

Get GHC 7.10.2 (MinGHC on Windows) and make sure that happy is installed. On linux you may need to install a package like libtinfo-dev to make the Haskell terminfo package work.

Now run the following to install the current snapshot of the master branch:

$ cabal install
$ ghcjs-boot

Haskell to JavaScript compiler

GHCJS is a Haskell to JavaScript compiler that uses the GHC API.

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.)
  • Cost-centres, stack traces
  • Cabal support, GHCJS has its own package database

And some JavaScript-specific features:

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


GHCJS can be installed with GHC 7.10.2 or later.


  • GHC 7.10.2 or higher
  • Cabal 1.22.4 and cabal-install 1.22.3 or higher
  • alex and happy
  • node.js 0.10.28 or higher. GHCJS uses node.js for its build system and for running Template Haskell.

Platform-specific preparation

Linux / OS X

  • A recent version of alex and happy need to be in your PATH
  • git, make, cpp, autoreconf need to be in your PATH
  • One of the dependencies is the terminfo Haskell package, which requires libtinfo. On Debian/Ubuntu this is provided by the libtinfo-dev package.


  • You need a shell that's capable of running autotools scripts (with git, make, cpp, autoreconf installed). See the GHCJS Wiki or the file for instructions for setting up MSYS2 for this.
  • A recent version of alex and happy need to be in your PATH
  • Virus scanners often interfere with configure scripts (permission denied errors), disable on-access scanning before running ghcjs-boot.

Installation steps

Install GHCJS (for compiler development)

Get ghcjs from Github and install it:

$ git clone
$ cabal install ./ghcjs

Build the libraries

Use ghcjs-boot to build the base libraries for GHCJS:

if you used the Git repository to install:
$ ghcjs-boot --dev

if you are doing a development build from a different branch than `master`, you need to tell `ghcjs-boot` to use the correct branch of the `ghcjs-boot` and `shims` repositories:
$ ghcjs-boot --dev --ghcjs-boot-dev-branch somebranch --shims-dev-branch

if you are installing from hackage, the ghcjs source distribution package already includes the correct libraries. run:
$ ghcjs-boot

Some distros install node.js as nodejs instead of node. Add --with-node nodejs to the ghcjs-boot command in that case.


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!

Cabal support

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.


You can use Cabal sandboxes with GHCJS, create a new sandbox with:

$ cabal sandbox init

Then you can just configure with --ghcjs to build with GHCJS inside the sandbox:

$ cabal install --ghcjs

If you also want to set GHCJS as the default compiler in the sandbox, run:

$ cabal sandbox init
$ echo "compiler: ghcjs" >> cabal.config

Setting the default compiler to ghcjs makes cabal sandbox exec and cabal sandbox hc-pkg use GHCJS-specific settings. These commands do not know about the configure flags, so setting the default compiler is the only way to make them use the correct settings for GHCJS.

Package databases

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 will 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!). Read the HACKING.markdown document to get started. The wiki may also contain useful information.

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 interface 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