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Haskell to Javascript translator

Project aims to provide solution to

  • compile modern Haskell libraries to Javascript files and use them in Ajax applications or
  • develop entire Ajax application in Haskell language

Previous version of project is located at


Code builds as standard haskell package

$ runghc Setup configure
$ runghc Setup build
$ runghc Setup install


To compile Haskell module to Javascript use ghcjs command.

$ ghcjs Test.hs

This command is merely equivalent to the following

$ ghc --make Test.hs

but it compiles to Javascript instead of native code.

See examples folder for an example of loading and running haskell code from browser.


The code is in alpha stage. Feel free to experiment with it as you wish.


Compiler is implemented as GHC backend using GHC API. And been tested with GHC 6.12.1.

Building Prelude

To play with any Haskell code you'll need Haskell standard library. GHC implements standard library as a "base" package. You'll need to compile modules from base package.

For now we only support 32-bit GHC for building base libraries. We emulate 32-bit integers in Javascript and we need base libraries to be build in 32-bit environment. You can build libraries on 32-bit system and use compiled Javascript on 64-bit system.

  1. Download ghc source distribution for the same version of ghc that you use to build ghcjs.

  2. Customize build

    $ cd ghc-x.xx.x
    $ cd mk
    $ cp

    Edit mk/

    Add the following line to the top:

    INTEGER_LIBRARY = integer-simple

    Set build flower to the quikest: uncomment the line:

    BuildFlavour = quickest
  3. Build ghc

    $ ./configure
    $ make
  4. You should try to use BuildPackages.hs script. You can build ghc-prim, integer-simple and base packages in examples directory with the following command

    $ cd examples
    $ ./BuildPackages.hs <path-to-ghc-directory>

    You can build packages manually using instructions below.

=== Building ghc-prim

$ cd ghc-x.xx.x
$ cd libraries/ghc-prim
$ ghcjs -odir <javascript files folder>/ghc-prim -hidir <javascript files folder>/ghc-prim -cpp -fglasgow-exts -package-name ghc-prim GHC/Types.hs 
$ ghcjs -odir <javascript files folder>/ghc-prim -hidir <javascript files folder>/ghc-prim -cpp -fglasgow-exts -package-name ghc-prim GHC/*

=== Building integer-simple

$ cd ghc-x.xx.x
$ cd libraries/integer-simple
$ ghcjs -odir <javascript files folder>/integer-simple -hidir <javascript files folder>/integer-simple -cpp -fglasgow-exts -package-name integer-simple GHC/Integer.hs 

=== Building base

$ cd ghc-x.xx.x
$ cd libraries/base
$ ghcjs -odir <javascript files folder>/base -hidir <javascript files folder>/base -hide-package base -package-name base -I./include -i./dist-install/build -XMagicHash -XExistentialQuantification -XRank2Types -XScopedTypeVariables -XUnboxedTuples -XForeignFunctionInterface -XUnliftedFFITypes -XDeriveDataTypeable -XGeneralizedNewtypeDeriving -XFlexibleInstances -XStandaloneDeriving -XPatternGuards -XEmptyDataDecls -XNoImplicitPrelude -XCPP Prelude.hs

This last magic command line was guessed using

cabal build -v

to see what options are passed to GHC.

We should really use

-odir /tmp

option to get read of useless object files, but it seems to be a bug in ghc that cause GHC to rely on odir to be the same as hidir wich is mostly the case in "normal" GHC usage.

Differences from old version contains the first implementation of the idea. This version is a rewrite that inherit little code from previous version. The differences are the following.

New features

  • It is more modular.

    But not enough. Generator.Core module needs some refactoring to separate some parts into stand alone modules.

  • Generated Javascript code is pretty readable and ideomatic.

  • Lazy module loading is implemented.

    Modules are loaded (using XMLHttpRequest) on demand when they are needed.


  • Foreign function interface is not supported. Only minimum of primitive operations is supported.

  • Tail recursion optimization is off by default.


    $ ghcjs --calling-convention=trampoline

    To optimize tail recursion in resulting code.