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

Quick Start - Developing GHCJS

GHCJS contains a library, ghcjs, which contains the JavaScript code generator and a slightly customized variant of the ghc library, and several executable programs.

The repository has several submodules and some files must be generated before the package can be installed.



You need the same major version of GHC as the version of the GHCJS branch you're building.


cabal-install 3.0 is supported

emscripten emsdk

GHCJS uses a C toolchain, mostly for build system related tasks like the C preprocessor, Autoconf scripts and tools like hsc2hs. Direct support for using compiled foreign libraries from Haskell code may follow at a later date.

Please follow the installation instructions at

GHCJS requires the "upstream" emscripten backend, which is the default now. The earlier "fastcomp" backend will not work.

getting and preparing the source tree

$ git clone
$ cd ghcjs
$ git submodule update --init --recursive

building the compiler

GHCJS depends on a few "local" packages in the source tree. You can use cabal-install and stack to set up a build environment that contains these packages.

Cabal new-install

After the source tree has been prepared, the package can be installed. You may want ensure that binaries of earlier versions are overwritten:

cabal v2-install --overwrite-policy=always --install-method=copy --installdir=inplace/bin

At the time of writing, cabal-install does not support creating symbolic links on Windows, even though this is the default installation method. A workaround is telling it to copy the executables instead:

cabal v1-install --prefix=inplace

v1 style Cabal sandbox

v1 style cabal sandboxes are also supported

if you want to build with a Cabal sandbox, use the script to add the local packages.

$ cabal v1-sandbox init
$ cabal v1-install


or you can use stack:

$ stack --system-ghc --skip-ghc-check install --local-bin-dir=inplace/bin

Booting GHCJS

The ghcjs-boot program builds the "boot" libraries, like ghc-prim, base and template-haskell with GHCJS. After booting, GHCJS can compile regular Haskell programs and packages.

ghcjs-boot needs to be able to find the emscripten toolchain, a nodejs executable. The easiest way to do this is by running the script. After that, you can run ghcjs-boot by pointing it to the boot libraries (the directory containing the boot.yaml file)

$ source ~/emsdk/
$ ./inplace/bin/ghcjs-boot -s ./lib/boot

GHCJS executables and library paths

After booting, you can add the directory containing the GHCJS binaries to your executable PATH. The ghcjs-boot program prints the location after finishing building the libraries.

You can also create a symbolic link for the ghcjs and ghcjs-pkg programs, or use the --with-compiler and --with-hc-pkg flags when using cabal-install

Generating a source distribution

if you work on boot packages that need some for an upstream library, make sure to update the patches in /lib/patches first

$ ./utils/

then regenerate the packages

$ ./utils/


Haskell to JavaScript compiler, based on GHC







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