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JavaScript backend merged into GHC

ยท 20 min read

A new JavaScript backend was merged into GHC on November 30th, 2022! This means that the next release of GHC will be able to emit code that runs in web browsers without requiring any extra tools, enabling Haskell for both front-end and back-end web applications.

In this post, we, the GHC DevX team at IOG, describe the challenges we faced bringing GHCJS to GHC, how we overcame those challenges, and what's left to do. This post is rather long so we've provided these links in case you would like to skip ahead:

Take me to the future of GHCJS
Tell me what to expect
Show me the product roadmap
Tell me how I can help
Just show me how to hello world! (Skip to build instructions)

Why JavaScript? Or, the Big Picture.โ€‹

To put it simply, the number of users on the internet is as low as it will ever be right now, and it is almost guaranteed that those users use JavaScript. At time of writing, JavaScript holds 97.3% of client-side programming market share (not to mention market share of front-end technologies). Furthermore, JavaScript is not going to disappear anytime soon. As more and more interactivity is pushed onto the internet, JavaScript will become more entrenched because of backwards compatibility, network effects and the amount of capital already devoted to it. JavaScript, like C and COBOL will be with us for the foreseeable future. This makes JavaScript an attractive target; it provides portability, allows us to capitalize on the massive investments in the language and platform, and essentially eliminates the risk that the we build our technology atop a disappearing or deprecating foundation.

WebAssembly is a promising target as well, and Tweag has just merged a WebAssembly backend into GHC (great work and congrats!). WebAssembly is not as ubiquitous as JavaScript yet, and has a harder time interacting with JavaScript directly. Hence, we believe that the WebAssembly and JavaScript backends provide different strengths, and it is to the Haskell community's benefit to have and support both code generation paths in GHC for different use cases and requirements.

Why Haskell?โ€‹

JavaScript has many problems ranging from the downstream effects of early design decisions (that inhibit programmer productivity and are subtle bug generators), to ecosystem security issues, to fundamental issues with asynchronous and concurrent programming.

These issues are problematic for our product domain. At IOG, a central engineering requirement is to create a code base that has a high degree of correctness. Haskell makes this easy; or to get a little technical, the combination of Strong Static Hindley-Milner based typing allows us to write performant, correct, and maintainable code. In addition to this, many of the problems that occur in JavaScript are simply not expressible because of Haskell's type system and concurrency offerings.

There are, of course, competitors: PureScript targets Javascript and provides a programmer experience close to Haskell's. The benefit of using Haskell instead is code sharing: we can write the front-end of a web app in Haskell that compiles to JavaScript and the back-end in Haskell that compiles to machine code. In particular, the (de)serialization code (e.g. from/to JSON) is shared and cannot get out of sync between the front-end and the back-end.

Why a GHC backend?โ€‹

Haskell is a language driven by its implementation in GHC. GHC development is very active and GHC does not define a stable interface for compiler backends that are independently maintained, which means that maintaining an out-of-tree backend is costly.

The maintenance burden is not hypothetical; our teammate Luite Stegeman has been developing a fork of GHC that emits JavaScript, called GHCJS, for close to 10 years and has experienced the pain first hand. Any changes to upstream GHC had to be adapted to the customized fork or GHCJS would fall behind. And fall behind it did: at the time of writing, GHCJS has stuck to using GHC 8.10, lagging behind by three major releases and counting.

Similarly, the Eta compilerโ€”which is targeting the JVMโ€”faced the same issues and appears to be discontinued (compatibility with GHC 7.10.3's Haskell from 2015 is mentioned).

Compounding the issue, the normal Haskell toolchain was not designed for an edge case like GHCJS. So GHCJS required that the normal tooling, e.g., Cabal and Stack, could distinguish between GHC and GHCJS compilers. This meant that the GHCJS developers had to maintain the GHC fork, develop GHCJS, and patch or contribute to Cabal and Stack. Simply put, the maintenance burden was much too high per developer. Examples of differences between GHCJS and GHC:

  • GHCJS had a double versionโ€”its own version and the version of GHC it was based onโ€”and build tools had to deal with both
  • GHCJS used non-standard file extension (e.g. .js_o and .js_a for objects and static libraries respectively) and custom file formats (still true for .o but no longer true for .a)

So instead of spending engineering time and energy responding to ecosystem changes and maintenance, the DevX team decided the best course of action was to enhance GHC's cross-compilation support and add a proper JavaScript backend based on GHCJS. We feel that this adds value to the entire Haskell ecosystem, keeps the JavaScript backend in sync with GHC, provides a better user experience for all, reduces maintenance costs, and greatly improves the backends in GHC in general. By implementing support for a JavaScript backend in GHC, we also improve GHC's support for cross-compilation (and testing cross-compilers), which is directly applicable to the WebAssembly, iOS, and Android backends in GHC.

Is GHCJS Dead?โ€‹

Not yet! As it stands, the JavaScript backend doesn't provide all the features provided by GHCJS. In particular it doesn't support Template Haskell and we've removed the extended GHCJS FFI syntax to refine its design. See our roadmap below for more details.

Nevertheless GHCJS is unlikely to be updated to use a GHC version more recent than 8.10.x. So from our point of view it is in maintenance mode until the JavaScript backend totally subsumes its features. New maintainers who want to continue the development of GHCJS until its feature set has been fully subsumed by mainline GHC are of course welcome.

What is Missing From GHCJS?โ€‹

The JavaScript backend borrows a lot of code from GHCJS, but not all of it. Here are the main differences between GHCJS and the JavaScript backend:

  1. GHCJS was stuck on GHC version 8.10 while the JavaScript backend follows GHC HEAD.

  2. GHCJS's incremental linking support ("base" bundles) hasn't been ported. This feature required too many changes (such as adding new command-line flags) and would have been backend-specific. This might be implemented in the future if it proves to be useful for the newer Template Haskell implementation, for example.

  3. GHCJS's JavaScript code optimizer hasn't been ported. The code was trying to do too much all at once and consequently was fragile and slow. We plan to work on an intermediate representation between STG and JavaScript to perform the same optimizations with better performance, maintainability, and reliability.

  4. GHCJS's compactor (link time optimizations) code hasn't been ported. Some optimizations have been reimplemented (e.g. global renaming of local identifiers), but some other are lacking (e.g. compacting initialization code). We plan to work on this as part of a larger effort on refactoring the code generator, the linker, and some aspects of the runtime system. More details are available in GHC issue #22352.

  5. GHCJS's hacky support for plugins hasn't been ported. Instead we implemented a new way to load plugins from shared libraries that works in any GHC cross-compiler. See #20964 and !7377.

    The common and convenient approach to load plugins still isn't supported by GHC when it is used as a cross-compiler (see #14335 for more details).

  6. GHCJS's support for Template Haskell hasn't been ported. GHCJS had its own implementation of an external interpreter (THRunner) which has been used as an inspiration to implement GHC's external interpreter (IServ). While serving the same purpose, IServ is quite different from THRunner and can't be directly used as a substitute for it. Retrofitting THRunner into Iserv is our next priority. More details on

  7. GHCJS supported an extended FFI import syntax allowing Javascript code to be inlined (the FFI import string supports templates of Javascript code with placeholders for arguments). This hasn't been ported because adding a JavaScript parser to GHC was difficult and complex, and the imported code made no safety guarantees whatsoever. For now, only JavaScript function calls are supported.

  8. Any command-line flag introduced by GHCJS has not been ported. We didn't make any change to GHC's command line in this work except for adding a -ddump-js flag. Other options will be added later as needed.

  9. The JavaScript backend itself hasn't been optimized and we even removed some undocumented uses of NFData from GHCJS's code. We intend to optimize the JavaScript backend in a principled way (e.g. by first gathering evidence with profiling).

What's on the JS Backend's Roadmap?โ€‹

Our top priorities are:

  • Implementing Template Haskell support.
  • Reducing generated JavaScript code size.
  • Modernizing the generated JavaScript code. The code generator adapted from GHCJS does not use more modern JavaScript features such as fat-arrows (=>), symbols and let bindings. We aim for the JavaScript backend to emit JavaScript that comports with ECMA-262.
  • Enhancing the run-time performance of the generated code

What has Improved Compared to GHCJS?โ€‹

Or, why did it take you so long to port a stripped GHCJS into GHC? While it may seem like such a task should be relatively quickโ€”especially in a language with such a good refactoring story like Haskellโ€”there were numerous road blocks that we needed to remove before adding the backend. In particular, here were the troublesome bits:

Removing the Use of External Librariesโ€‹

GHCJS used libraries that aren't already dependencies of GHC, such as text, lens, attoparsec, and aeson. As we didn't want to add new dependencies to GHC, we've refactored the code to avoid them. Examples:

  • we've replaced Text with GHC's ShortText (which provides a similar API) and finally with GHC's FastString in most cases (which is usually more performant).
  • we've replaced a lot of lens-heavy code with its non-lens equivalents, because GHC does not use lenses itself, and a design requirement was to stay within existing code conventions.
  • we've replaced pretty with GHC's pretty-printer (SDoc, etc.).
  • we've replaced binary with GHC's Binary instances.

GHCJS used to provide its own base and prim libraries: ghcjs-base and ghcjs-prim. We've merged those into the existing base and ghc-prim libraries.

Reusing GHC's Build System: Hadrianโ€‹

GHCJS has a reputation for being complex to build. It relied on custom build scripts to deal with the GHC fork it uses. The JavaScript backend however is as easy to build as any other GHC. It doesn't require any wrapper script, only the emconfigure tool provided by the Emscripten project.

With a fresh checkout of the GHC source tree, you can now build a GHC with the JavaScript backend with just these commands:

> ./boot
> emconfigure ./configure --target=js-unknown-ghcjs
> ./hadrian/build --bignum=native -j

Note that if this doesn't work, up to date instructions and troubleshootings can be found on

The Hadrian build system has been adapted to support Cabal's js-sources stanzas that are to support user-provided .js files. Both the rts and base packages required this feature.

Support for Running GHC's Test Suiteโ€‹

GHC's entire test suite can now run and check the JavaScript backend! We had to tweak Hadrian to make this possible (to make Hadrian cross-compiler aware), but the test suite has already found some bugs that we have since fixed.

However, in order to merge for the GHC 9.6 release we had to disable many tests because of missing features (Template Haskell, Haskell Program Coverage (HPC), compact regions, etc.) or because the generated code would time out (not surprising given the missing optimizer and compactor).

But in the process of disabling those tests we've laid a good path forward. We've added more precise properties to the test suite, which indicate the required features to run each test. So when we implement some feature, it will be painless to re-enable all its tests. In addition, failing tests now have proper tickets in GHC's issue tracker.

We've spent some time trying to run the test suite on CI but this work wasn't ready in time to be included in the initial commit with the rest of the backend. For now, only some basic testing is done on CI: compiling a non trivial program that uses the GHC library into JavaScript and executing it. Nevertheless, we have a merge request in the works so that future contributions should be properly validated by running the test suite on CI soon.

For the time being, the following command will run the test suite locally:

./hadrian/build --bignum=native -j2 test

We use -j2 to avoid running too many tests in parallel as this could allocate too much memory and fail, which isn't surprising as the JavaScript backend hasn't been optimized for memory usage yet.

Upgrading from GHC 8.10 to GHC 9.6โ€‹

The latest version of GHCJS is based on a fork of GHC 8.10.7. We spent a significant amount of time adapting the code generator to support GHC HEAD. In practice this meant:

  • Adding support for new primops, especially sized primitives.
  • Adapting to ghc-bignum changes.
  • Adapting to internal changes.
  • Fixing support for polymorphism in kinds.
  • Fixing support for unlifted newtypes.
  • Fixing support for unboxed sums.
  • Many other fixes...

Fixing Some Performance Issuesโ€‹

As we haven't ported GHCJS's Compactor, output size was predictably incredibly large. So we've spent time re-implementing a crucial piece of the Compactorโ€”renaming and shortening of local variablesโ€”using a different approach. Our new approach ended up being faster than GHCJS's compactor. For the GHC devs out there, as we first replaced the Text type with the FastString type, the newer Compactor can now replace a FastString-based identifier with a new identifier derived from the FastString's Unique in constant time.

Removal of Custom File Extensions and Support for JavaScript Pragmasโ€‹

GHCJS used the .js.pp file extension to identify JavaScript files that needed to be passed through CPP before being valid JavaScript. Adding support for this extension in both Hadrian and GHC proved to be more work than just adding support for JavaScript pragmas. So we decided to do the latter; similarly to Haskell extension pragmas, you can now write //#OPTIONS: CPP in your JavaScript files to enable the CPP pass, and the file extension is always .js.

While we're on the topic of file extensions, technically .js files don't have to be compiled into .o files (contrary to C/C++/Haskell/etc. files) at all. However, build systems (Hadrian, Cabal...) and compilers (GHC) expect this. So for consistency with other backends, we've added a fake compilation pass for .js files too. They are now renamed into .o files with a //JAVASCRIPT header added to distinguish them from object files produced by the JavaScript backend (and from Emscripten, in the future).

Cleanup and Documentationโ€‹

GHC provides some utilities (pretty-printer, binary serialization, string interning, etc.) that GHCJS did not make use of. So we adapted the GHCJS code to exploit these utilities, keep the JavaScript backend similar to other backends, and for better performance.

Three of us (out of four) were totally new to GHCJS's code base. We strived to grok the code and to make it understandable by adding a lot of comments and refactoring. Throughout this process we logged our learning in our engineering blog to explain some (sadly not all) technical details about GHCJS's internals:

Plugin Support in Cross-Compilersโ€‹

GHC doesn't support plugins when built as a cross-compiler (cf #14335). This is because it cannot yet support two environments at once: one for the target code (JavaScript code here) and one for the host (e.g. native x86 or AArch64 code for the plugin). We've spent a lot of time making it more modular (see the Modularizing GHC white paper we published earlier this year and Sylvain's lightning talk at HIW 2022) but there is a lot more to do to achieve this (cf #17957).

GHCJS used a fragile hack to support plugins: at plugin loading time it would substitute the plugin unit with another corresponding one from another package database (For the non-GHC devs out there interested in GHC Units see this note). This was fragile because it could violate GHC's single environment assumptions.

GHCJS's hack did not get ported. Nevertheless we have implemented a new way for GHC to load plugins directly from libraries instead of packages (#20964/!7377). This method doesn't require GHC to load module interfaces for the plugin and its dependencies, hence workarounds GHC's limitations.

What About Libraries Using C Sources?โ€‹

Libraries that use C sources (c-sources Cabal stanza) aren't supported by the JavaScript backend. In the future we plan to use Emscripten to compile C sources and then to generate some adapter code for them, but this isn't done yet.

For now, there are two ways to fix libraries that use C sources. The C code can either be rewritten in Javascript, or it can be rewritten in Haskell. Then it is possible to use Cabal predicates (e.g. arch(js)) to select between the different versions.

We do have a preference for writing pure Haskell versions because it is more future proof. For example if someone adds some new backends for Lua, Java, CLR, etc. then the Haskell version can be directly compiled by that backend and there is no extra work. In contrast, if the C source is rewritten in JavaScript, then it would need to be rewritten for each backend.

That is the approach we've taken when we wrote the ghc-bignum library. Ghc-bignum provides a "native" implementation written in Haskell that is functionally equivalent to the GMP based implementation. Of course, besides being more future proof the Haskell version is just more pleasant to write than the Javascript version.

Note that GHCJS came with a "shim" library where a shim is JavaScript source code specifically for some package. For example, GHCJS provided shims for packages like text, process, and hashable. We do not intend the JavaScript backend to provide shims so these JavaScript sources will have to be upstreamed or reimplemented in Haskell.

Note that the linking behavior is different due to the interpreted nature of Javascript. In the JavaScript backend, we can link with libraries using foreign imports even if the imported functions don't exist. Instead of failing at link time (which is what usually happens with native code) a JavaScript exception is raised only when and if the imported function is called.

How to Help?โ€‹

We have now reached our first milestone; anyone can easily build and test the JavaScript backend, and anyone can open bug reports or offer patches for the JavaScript backend on GHC's GitLab.

For those who offered their help this year: thank you! Until now it was difficult to split the work into independent tasks (one fix led to a new failure, which led to an architectural issue, etc.) and it was difficult to coordinate with people outside of our team. However, we're now in a much better position to discuss suggestions and to test/review patches in the spirit of open source.

tl;dr Just Tell Me How to Say Hello Worldโ€‹

You need:

  • Emscripten version 3.14 or better. Be sure that your emscripten is bundled with either LLVM 15 or an up to date, patched LLVM 14.
  • Nodejs, latest stable version. Only if you want to run the compiled JavaScript with node.

Most Linux distributions will have the necessary LLVM patches. If you're on NixOS, you'll need to use llvm_git and hope for the best. This fork of ghc.nix will also be useful to you.

Checkout the GHC sourceโ€‹

git clone --recurse-submodules
cd ghc # ensure you are in the ghc source tree for the following commands

Update the submodulesโ€‹

git submodule update --init --recursive

Boot and Configure for JavaScriptโ€‹

./boot && emconfigure ./configure --target=js-unknown-ghcjs

You should see configure finish and report something similar:

Configure completed successfully.

Building GHC version : 9.5.20220819
Git commit id : 08c3c4783c72d3173d79ccda2ac282e2d3e04e34

Build platform : x86_64-unknown-linux
Host platform : x86_64-unknown-linux
Target platform : js-unknown-ghcjs

Bootstrapping using : /nix/store/4bkmkc7c98m4qyszsshnw9iclzzmdn4n-ghc-9.2.3-with-packages/bin/ghc
which is version : 9.2.3
with threaded RTS? : YES

Using (for bootstrapping) : /nix/store/yzs8390walgk2rwl6i5li2g672hdn0kv-gcc-wrapper-11.3.0/bin/cc
Using clang : /nix/store/p894nlicv53firllwgrfxfi51jzckh5l-emscripten-3.1.15/bin/emcc
which is version : 15.0.0
linker options :
Building a cross compiler : YES
Unregisterised : NO
TablesNextToCode : YES
Build GMP in tree : NO
hs-cpp : /nix/store/p894nlicv53firllwgrfxfi51jzckh5l-emscripten-3.1.15/bin/emcc
hs-cpp-flags : -E -undef -traditional -Wno-invalid-pp-token -Wno-unicode -Wno-trigraphs
ar : /nix/store/p894nlicv53firllwgrfxfi51jzckh5l-emscripten-3.1.15/bin/emar
ld : /nix/store/p894nlicv53firllwgrfxfi51jzckh5l-emscripten-3.1.15/bin/emcc
nm : /nix/store/0dp0bfg9sncg7bjy389zwyg2gskknm6b-emscripten-llvm-3.1.15/bin/llvm-nm
objdump : /nix/store/zgvxnf9047rdd8g8kq2zxxm9k6kfqf8b-binutils-2.38/bin/objdump
ranlib : /nix/store/p894nlicv53firllwgrfxfi51jzckh5l-emscripten-3.1.15/bin/emranlib
otool : otool
install_name_tool : install_name_tool
windres :
dllwrap :
genlib :
Happy : /nix/store/ijdmyaj6i6hgx5ll0lxxgcm9b0xn8nma-happy-1.20.0/bin/happy (1.20.0)
Alex : /nix/store/qzgm2m7p7xc0fnyj4vy3jcmz8pvbg9p7-alex-3.2.6/bin/alex (3.2.6)
sphinx-build : /nix/store/27dk5i52465a4azjr2dqmrhyc0m4lpf2-python3.9-sphinx-4.5.0/bin/sphinx-build
xelatex : /nix/store/8jc2258h4nqzqjy303zzkssd3ip675pf-texlive-combined-2021/bin/xelatex
makeinfo : /run/current-system/sw/bin/makeinfo
git : /nix/store/vsr2cn15h7cbwd5vqsam2ab2jzwfbyf9-git-2.36.0/bin/git
cabal-install : /nix/store/cjmd2qv1b5pdw4lxh1aw4xwwy4ibnb2p-cabal-install-

Using LLVM tools
clang : clang
llc : llc
opt : opt

HsColour was not found; documentation will not contain source links

Tools to build Sphinx HTML documentation available: YES
Tools to build Sphinx PDF documentation available: YES
Tools to build Sphinx INFO documentation available: YES

Be sure to verify that ar, ld, nm and friends point to the emscripten versions, i.e., the output shows <tool> : <some-path>-emscripten-<tool>.

Build the JavaScript backendโ€‹

./hadrian/build --bignum=native -j

Now Compile Hello Worldโ€‹

module Main where

main :: IO ()
main = putStrLn "Hello JS!"
$ <path-to-ghc-root-dir>/_build/ghc-stage1 -fforce-recomp Main.hs
$ ./Main
$ Hello JS!

Under the hood Main is just a JavaScript program written as a script with nodejs as the interpreter. This means you can treat the compiled program like any other JavaScript program: loading it into JavaScript tooling or hack on it by hand. This also means that all compiled programs, such as Main, are human-readable, for example here are the first ten lines:

$ head Main
#!/usr/bin/env node
var h$currentThread = null;
var h$stack = null;
var h$sp = 0;
var h$initStatic = [];
var h$staticThunks = {};
var h$staticThunksArr = [];
var h$CAFs = [];
var h$CAFsReset = [];
var h$regs = [];

The program begins with a shebang instructing the operating system to send the rest of the file to nodejs. The remaining lines are our actual program, which starts with global variables that the runtime system, garbage collector, and scheduler need. Now human-readable is not the same as easy to understand, for example here is the logic that implements a Maybe:

function h$baseZCGHCziMaybeziJust_con_e() { return h$rs() };
function h$baseZCGHCziMaybeziJust_e() {
var h$$13be2042 = h$r2;
h$r1 = h$c1(h$baseZCGHCziMaybeziJust_con_e, h$$13be2042);
return h$rs();
function h$baseZCGHCziMaybeziNothing_con_e() { return h$rs() };

If you would like to understand this code and how the JavaScript backend works in general please see our other blog posts. In any case, we invite you to try it out, hack, and be merry!


We want to thank Jan Hrcek, and David Thrane Christiansen for their time, labor, comments, and suggestions on drafts of this blog post.