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2 min read

This is the July 2022 monthly update from the GHC DevX team at IOG.

JavaScript Backend for GHC

For a few months we have been merging GHCJS (Haskell to JavaScript compiler) into GHC. We set our first milestone to be the ability to compile and to run the usual "Hello World" program. This month we finally reached it!

We are now focusing on:

  • fixing failing tests in GHC's testsuite (~2800 unexpected failures). To do that, we have to implement new primops, to fix bugs we introduced while we ported the code from GHCJS, etc.

  • implementing support for the "js-sources" Cabal stanza in Hadrian. Currently the JS backend finds the JS sources required for the RTS and for base into explicitly defined location. It was only a stop-gap measure and we now need to implement proper support for user-provided JavaScript files.

  • documenting and refactoring the source code and making it similar to other GHC modules. As an example, GHCJS used the text package which isn't a boot package. Hence we first switched to use GHC's ShortText implementation and now we switched to a FastString based implementation.

  • adding back GHCJS's features that we haven't ported for some reasons (e.g. the compactor, TH, etc.).

You can follow our progress on our development branch here.

Blog posts

For the time being, we will focus blog post topics on GHCJS internals and related topics. A few of these blog posts are currently under review and should be published shortly.

2 min read

This is the June 2022 monthly update from the GHC DevX team at IOG.

JavaScript Backend for GHC

For a few months we have been merging GHCJS (Haskell to JavaScript compiler) into GHC. We set our first milestone to be the ability to compile and to run the usual "Hello World" program. It turned out to be much more involved than we initially thought (requiring FFI support, etc.), but we should be getting there soon.

This month we have made the following progress:

  • Linking: GHCJS requires some functions to be directly implemented in JavaScript (e.g. the RTS, some low-level functions in base). We have added support for linking .js files. We've also added support for a preprocessing pass with CPP for .js.pp files.

  • js-sources: there is some ongoing work to load these external JavaScript files from installed libraries. Cabal provides a js-sources stanza for this, we need to adapt Hadrian to make use of it.

  • Binary vs Objectable: GHCJS used its own ByteString-based Objectable type-class: we replaced it with GHC's similar Binary type-class. Josh has published a blog post about their differences.

  • 64-bit primops: we've added support for 64-bit primops (Word64# and Int64# types). In GHCJS (GHC 8.10), these were still implemented as foreign function calls. It's no longer true on GHC head.

  • base library: added CPP as required to support the JS backend. Ported and converted FFI imports from GHCJS to use JavaScript fat arrows (we haven't implemented GHCJS's fancy import syntax yet).

Now we can compile and link the "HelloWorld" program. To reach the first milestone we only have to fix the remaining runtime errors.

You can follow our progress on our development branch here. We now rebase this branch every Friday to avoid lagging too much behind GHC head.

Haskell Optimization Handbook

The "Haskell Optimization Handbook" is an accepted proposal of the Haskell Foundation. Jeff has been steadily writing some initial material as per the project plan.

3 min read

This is the May 2022 monthly update from the GHC DevX team at IOG.

JavaScript Backend for GHC

For a few months we have been merging GHCJS (Haskell to JavaScript compiler) into GHC. We set our first milestone to be the ability to compile and to run the usual "Hello World" program. It turned out to be much more involved than we initially thought (requiring FFI support, etc.), but we should be getting there soon.

This month we have made the following progress:

  • RTS: we have modified Hadrian and rts.cabal in order to build a valid native rts unit that GHC can use, in particular containing appropriate header files.

  • linker: the JS linker has been hooked up with GHC's driver. We fixed several panics in the linker due to erroneous symbol generation code. These bugs were introduced while porting the code from the old 8.10 pretty-printing infrastructure to the newer one.

  • boot libraries: the JS backend can now build and link all the boot libraries. Note that we are not claiming that they are all usable yet. In particular complete FFI support is lacking, but the JS backend Hadrian build completes and so we can start using the produced JS cross-compiler.

  • levity polymorphism: building ghc-prim uncovered a lurking bug related to levity polymorphism. It wasn't noticed in GHCJS 8.10 because it is also related to the BoxedRep proposal that introduced a constructor application in a commonly used RuntimeRep.

  • sized literals: support for new sized literals have been added to the code generator.

Now that have achieved a build process that actually produces a JS cross compiler, we are confronting and fixing issues in the produced JavaScript code, such as adding, managing, and debugging CPP conditional compilation blocks in JS shim files. You can follow our progress on our development branch here.

External Static Plugins

GHC doesn't support plugins in cross-compilers #14335. Some time ago, we came up with a solution called "external static plugins" !7377. These are plugins that are directly loaded from shared libaries, bypassing the issue with usual plugins.

Our colleague Shea Levy confirmed that the approach works, backported it to GHC 8.10, and has been working on making it work in stage1 cross-compilers for Windows. Kudos for this work, Shea.

As the current user-interface based on environment variables isn't convenient, we have been working on adding new command-line flags to GHC instead. We expect to propose this for integration into GHC when the new interface will be fully implemented.

Blog posts

Inspired by our friends and colleagues at Well-Typed and Tweag, we have been starting to write blog posts for IOG's engineering blog. They will mostly be about stuff we are working on or that we are interested in. Feel free to send us feedback about these posts and to send us topics you would be interested to read about.

Haskell Optimization Handbook

The "Haskell Optimization Handbook" is an accepted proposal of the Haskell Foundation. Jeff has been working behind the scene to make this proposal concrete. More about this in the upcoming months.

2 min read

Welcome to the (rather late) April 2022 monthly update from the GHC DevX team at IOG. Since the last update we've continued work on the upcoming JavaScript backend for GHC. Unfortunately, we have nothing to show quite yet but that doesn't mean nothing has happened! On the contrary, we've made great progress and are close to that crucial first milestone hello world. Besides our work on the JavaScript backend, we were pleased to finally push through the Modularizing GHC paper that Sylvain has been working on for 2+ years! It causes quite the splash on the Haskell discourse and reddit, we recommend reading it if you haven't already (links below). Alright, enough introduction let's get into the update.

JavaScript Backend

We have made the following progresses in the implementation of a JavaScript backend for GHC (adapted from GHCJS):

  • linker: ported GHCJS's linker code into GHC. A lot of code was duplicated from GHC and slightly modified for GHCJS's needs, making the process far from trivial.

  • testsuite: fixed Hadrian to run GHC's testsuite with cross-compilers !7850. There are remaining issues though (see #21292).

  • build system: fixes for GHC's configure script were ported (e.g. support for the "ghcjs" target in config.sub). GHCJS's custom build script was integrated into We can now configure the build with: ./configure --target=js-unknown-ghcjs

  • TH: we have conducted some experiments to find the best way to bridge GHCJS's TH runner and GHC's external interpreter. This will be described in details in a future blog post.

  • FFI: basic support for JavaScript FFI has been ported from GHCJS to GHC. We haven't ported the JavaScript parser, so we have dropped the fancy import syntax (e.g. "$"). It should be enough to build boot libraries and we will add JS parsing support later.

At this stage, we are working on building boot libraries and on supporting linking with the JS RTS.

Development happens in the following branch:

Modularity paper

Sylvain, Jeffrey, and John Ericson (from Obsidian Systems) wrote a paper about "modularizing GHC" using domain-driven design.

We've got a lot of great feedback about it (expect a first revision soon). We also got a GHC contribution directly inspired by the paper (see !8160) which was very welcome!

3 min read

JS Backend

In March the team focused on porting more GHCJS code to GHC head.

  • Most of us are new to GHCJS鈥檚 codebase so we are taking some time to better understand it and to better document it as code gets integrated into GHC head.
  • Development process: initially we had planned to integrate features one after the others into GHC head. However it was finally decided that features would be merged into a wip/javascript-backend branch first and then later merged into GHC head. After trying this approach we decided to work directly into another branch: wip/js-staging . Opening merge requests that can鈥檛 be tested against a branch that isn鈥檛 GHC head didn鈥檛 bring any benefit and slowed us too much.
  • Documentation: we wrote a document comparing the different approaches to target JavaScript/WebAssembly
  • RTS: some parts of GHCJS鈥檚 RTS are generated from Haskell code, similarly to code generated with the genapply program in the C RTS. This code has been ported to GHC head. As JS linking---especially linking with the RTS---will only be performed by GHC in the short term, we plan to make it generate this code dynamically at link time.
  • Linker: most of GHCJS鈥檚 linker code has been adapted to GHC head. Because of the lack of modularity of GHC, a lot of GHC code was duplicated into GHCJS and slightly modified. Now that both codes have diverged we need to spend some time making them converge again, probably by making the Linker code in GHC more modular.
  • Adaptation to GHC head: some work is underway to replace GHCJS鈥檚 Objectable type-class with GHC鈥檚 Binary type-class which serves the same purpose. Similarly a lot of uses of Text have been replaced with GHC鈥檚 ShortText or FastString.
  • Template Haskell: GHCJS has its own TH runner which inspired GHC鈥檚 external interpreter (鈥淚serv鈥) programs. We have been exploring options to port TH runner code as an Iserv implementation. The Iserv protocol uses GADTs to represent its messages which requires more boilerplate code to convert them into JS because we can鈥檛 automatically derive aeson instances for them.
  • Plugins: we have an MR adding support for 鈥渆xternal static plugins鈥 to GHC !7377. Currently it only supports configuring plugins via environment variables. We have been working on adding support for command-line flags instead.
  • Testsuite: we have fixed GHC鈥檚 build system so that it can run GHC鈥檚 testsuite when GHC is built as a cross-compiler (!7850). There is still some work to do (tracked in #21292) to somehow support tests that run compiled programs: with cross-compilers, target programs can鈥檛 be directly executed by the host architecture.


  • Performance book: some time was spent on the infrastructure (CI) and on switching the format of the book to ReStructured Text
  • Modularity: some time was spent discussing GHC鈥檚 design and refactoring (c.f. !7442 and #20927).

2 min read

JS backend

This month we worked on adapting code from GHCJS to merge into GHC head. We also started discussing the implementation process publicly and especially with our colleagues at Well-Typed.

  • Ticket about adapting GHCJS鈥 code into a proper JS backend for GHC has been opened [#21078]. Feedback was very positive!
  • There were discussions about the process and an agreement to target GHC 9.6 release [email on ghc-devs, wiki page]
  • deriveConstants is a program used to generate some header file included in the rts package. While it is mainly useful for native targets, we had to make it support Javascript targets [!7585]
  • Javascript is going to be the first official target platform supported by GHC that has its own notion of managed heap objects. Hence we may need a new RuntimeRep to represent these values for Haskell codes interacting with JS codes via FFI. We opened !7577 into which we tried to make this new RuntimeRep non JS specific so that it could be reused for future backends targeting other managed platforms (e.g. CLR, JVM). It triggered a lot of discussions summarized in #21142.
  • GHCJS鈥檚 code generator was ported to GHC head [!7573]. In its current state, we can generate Javascript unoptimised code -- the optimiser hasn鈥檛 been ported yet -- by compiling a module with -c -fjavascript. It required many changes, not only to adapt to changes between GHC 8.10 and GHC head but also to avoid adding new package dependencies. It was also an opportunity to refactor and to document the code, which is still a work in progress.
  • GHC doesn鈥檛 use any lens library, hence to port the code generator we had to replace lenses with usual record accessors. It turned out that case alternatives in STG lacked them because they were represented with a triple. We took the opportunity to introduce a proper record type for them !7652

Plutus-apps JS demo

  • We improved the proof of concept JavaScript library for generating Plutus transactions with a given set of constraints and lookups, exposing functionality from the plutus-ledger-constraints package. [Report]


9 min read

IOG is committed to improving Haskell developer experience, both by sponsoring the Haskell Foundation and by directly founding a team committed to this task: the Haskell DX team.

The team now tries to provide regular (monthly) updates about its work. This post is a bit longer because it covers all of 2021 which has not been covered anywhere else.

Code generation

  • Added a new backend for AArch64 architectures, especially to support Apple鈥檚 M1. Previously AArch64 was only supported via the LLVM based backend which is much slower. [!5884]
  • Added support for Apple鈥檚 M1 calling convention. In GHC 9.2.1 it implied making lifted sized types (e.g. Word8, Int16...) use their unlifted counterparts (e.g. Word8#, Int16#...); in GHC 8.10.7 鈥 a minor release 鈥撀 a less invasive but more fragile solution was implemented [commit].
  • Fixed a very old GHC issue [#1257] by making GHCi support unboxed values [!4412]: ByteCode is now generated from STG instead of directly from Core. It allows more Haskell codes to be supported by HLS and it even allows GHC code to be loaded into GHCi [link].
  • Fixed a bug in the Cmm sinking pass that led to register corruption at runtime with the C backend. Even if we don鈥檛 use the C backend, fixing this avoided spurious errors in CI jobs using it [#19237,!5755]
  • Fixed a register clobbering issue for 64-bit comparisons generated with the 32-bit x86 NCG backend [commit].
  • Fixed generation of switches on sized literals in StgToCmm [!6211]
  • Fixed LLVM shifts [#19215,!4822]


  • Fixed an off-by-one error in the MachO (Darwin) linker [!6041]. The fix is simple but the debugging session was epic!
  • Fix to avoid linking plugin units unconditionally with target code, which is wrong in general but even more so when GHC is used as a cross-compiler: plugins and target code aren鈥檛 for the same platform [#20218,!6496]


  • With John Ericson (Obsidian Systems) we finally made GHC independent of its target [!6791,!6539]. It means that there is no need to rebuild GHC to make it target another platform, so it now becomes possible to add support for a --target=... command-line flag [#11470]. It also means that a cross-compiling GHC could build plugins for its host platform in addition to building code for its target platform.
  • A side-effect of the previous bullet is that primops鈥 types are now platform independent. Previously some of them would use Word64 on 32-bit architectures and Word on 64-bit architectures: now Word64 is used on every platform. A side-effect of this side-effect is that we had to make Word64 as efficient as Word: it now benefits from the same optimizations (constant folding #19024, etc.). On 32-bit platforms, it reduced allocations by a fair amount in some cases: e.g. -25.8% in T9203 test and -11.5% when running haddock on base library [!6167]. We hope it will benefit other 32-bit architectures such as JavaScript or WebAssembly.
  • GHC built as a cross-compiler doesn鈥檛 support compiler plugins [#14335]. We have been working on refactoring GHC to make it support two separate environments in a given compiler session 鈥 one for target code and another for the plugin/compiler code. The implementation in [!6748] conflicts quite a lot with the support of multiple home-units that was added at about the same time. GHC needs to be refactored a lot more to correctly support this approach, so instead we implemented a different approach to load plugins which is more low-level and bypasses the issue [#20964, !7377].
  • We made GHC consider the target platform instead of the host platform in guessOutputFile [!6116]
  • Use target platform instead of host platform to detect literal overflows [#17336,!4986]


  • We updated GHCJS to use GHC 8.10.7 [branch]
  • We worked on making GHCJS鈥檚 codebase more suitable for integration into GHC: reducing the number of dependencies, avoiding the use of Template Haskell, reusing GHC鈥檚 build system, etc. There is now a GHCJS integrated into a GHC 8.10.7 fork [branch].
  • This experience led us to plan the realization of a JS backend into GHC head based on GHCJS. More information about this topic in our next report.
  • We worked on making GHC鈥檚 testsuite pass with GHCJS, triaging tests that legitimately fail on a JS platform from tests revealing real GHCJS issues. [LINK]


  • We seemed to be the first to try to build GHC on Windows with the updated GNU autotools 2.70 and this release made a breaking change to the way auxiliary files (config.guess, config.sub) were handled, breaking GHC鈥檚 build (#19189). The root cause of the issue couldn鈥檛 be easily solved so we modified GHC鈥檚 build system to avoid the use of these auxiliary files, bypassing the issue. Most GHC devs won鈥檛 ever notice that something was broken to begin with when they will update their GNU toolchain on Windows. [!4768,!4987,!5065]
  • Fixed cross-compilation of GHC from Linux to Windows using Hadrian [#20657,!6945,!6958]


  • Fixed Natural to Float/Double conversions to align with the method used for Integer to Float/Double and added missing rewrite rules [!6004]
  • Made most bignum literals be desugared into their final form in HsToCore stage instead of CoreToStg stage to ensure that Core optimizations were applied correctly to them [#20245,!6376]
  • Some constant folding rules were missing and were added:
  • Allowed some ghc-bignum operations to inline to get better performance, while still managing to keep constant-folding working [#19641,!6677,!6696,!6306]. There is some work left to do (cf #20361) but it is blocked by #19313 which in turn is blocked by #20554 which should be fixed soon (!6865, thanks Joachim!).
  • The ubiquitous fromIntegral function used to have many associated rewrite rules to make it fast (avoiding heap allocation of a passthrough Integer when possible) that were difficult to manage due to the combinatorial number of needed rules (#19907, #20062). We found a way to remove all these rules (!5862).

Technical debt & modularity

  • Made several component of the compiler independent of DynFlags (parsed command-line flags):
  • Made the handling of 鈥減ackage imports鈥 less fragile [!6586] and refactored some code related to dependencies and recompilation avoidance [!6528,!6346].
  • Abstracted plugin related fields from HscEnv [!7175]
  • Made a home-unit optional in several places [!7013]: the home-unit should only be required when compiling code, not when loading code (e.g. when loading plugins in cross-compilers #14335).
  • Made GHC no longer expose the (wrong) selected ghc-bignum backend with ghc --info. ghc-bignum now exposes a backendName function for this purpose [#20495,!6903]
  • Moved tmpDir from Settings to DynFlags [!6297]
  • Removed use of unsafePerfomIO in getProgName [!6137]
  • Refactored warning flags handling [!5815]
  • Made assertions use normal functions instead of CPP [!5693]
  • Made the interpreter more independent of the driver [!5627]
  • Replaced ptext . sLit with text [!5625]
  • Removed broken 鈥渄ynamic-by-default鈥 setting [#16782,!5467]
  • Abstracted some components from the compiler session state (HscEnv):
    • unit-related fields into a new UnitEnvdatatype [!5425]
    • FinderCache and NameCache[!4951]
    • Loader state [!5287]
  • Removed the need for a home unit-id to initialize an external package state (EPS) [!5043]
  • Refactored -dynamic-too handling [#19264,!4905]


  • Made divInt#, modInt# and divModInt# branchless and inlineable [#18067,#19636,!3229]
  • Fixed Integral instances for Word8/16/32 and showWord to use quotRemWordN [!5891,!5846]
  • Improved performance of occurrence analysis [#19989,!5977]
  • Fixed unnecessary pinned allocations in appendFS [!5989]
  • Added a rewrite rules for string literals:
    • Concatenation of string literals [#20174,#16373,!6259]
    • (++) . unpackCString# 鈬 unpackAppendCString# leading to a 15% reduction in compilation time on a specific example. [!6619]
    • Compute SDoc literal size at compilation time [#19266, !4901]
  • Fix for Dwarf strings generated by the NCG that were unnecessarily retained in the FastString table [!6621]
  • Worked on improving inlining heuristics by taking into account applied constructors at call sites [#20516,!6732]. More work is needed though.
  • Fixed #20857 by making the Id cache for primops used more often [!7241]
  • Replaced some avoidable uses of replicateM . length with more efficient code [!7198]. No performance gain this time but the next reader of this code won鈥檛 have to wonder if fixing it could improve performance.
  • Made exprIsCheapX inline for modest but easy perf improvements [!7183]
  • Removed an allocation in the code used to write text on a Handle (used by putStrLn, etc.) [!7160]
  • Replaced inefficient list operations with more efficient Monoid ([a],[b]) operations in the driver [!7069], leading to 1.9% reduction in compiler allocations in MultiLayerModules test.
  • Disabled some callstack allocations in non-debug builds [!6252]
  • Made file copy in GHC more efficient [!5801]
  • Miscellaneous pretty-printer enhancements [!5226]
  • Type tidying perf improvements with strictness [#14738,!4892]


  • Fixed issues related to the RTS鈥檚 ticker
    • Fixed some races [#18033,#20132,!6201]
    • Made the RTS open the file descriptor for its timer (timerfd) on Linux synchronously to avoid weird interactions with Haskell code manipulating file descriptors [#20618,!6902].
  • Moved GHC鈥檚 global variables used to manage Uniques into the RTS to fix plugin issues [#19940,!5900]

Build system / CI

  • Fixed Hadrian output to display warnings and errors after the multi screen long command lines [#20490,!6690]
  • Avoided the installation of a global platformConstants file; made GHC load constants from the RTS unit instead, allowing it to be reinstalled with different constants [!5427]
  • Made deriveConstants output its file atomically [#19684,!5520]
  • Made compression with xz faster on CI [!5066]
  • Don鈥檛 build extra object with -no-hs-main [#18938,!4974]
  • Add hi-boot dependencies with ghc -M [#14482,!4876]


  • Stack: fixed interface reading in hi-file-parser to support GHC 8.10 and 9.0 [PR, Stack#5134]
  • Enhanced pretty-printing of coercions in Core dumps [!4856]

One min read

Hopefully 2022 should be the year GHC will get a JavaScript backend without relying on GHCJS. This month the team has been busy planning the work that needs to be done to get there!


  • GHCJS has been updated to reduce the gap with GHC 8.10.7 codebase to the point that GHC鈥檚 build system is used to build GHCJS
  • Internal work planning for the integration of GHCJS into GHC
  • A different approach to load plugins into cross-compilers has been implemented [#20964, !7377]
  • GHCJS has been exercised to showcase compilation of some Plutus applications


  • A few 鈥渟ubsystems鈥 of GHC have been made more modular and reusable by making them independent of the command-line flags (DynFlags) [#17957, !7158, !7199, !7325]. This work resulted in a 10% reduction in call sites to DynFlags and has now removed all references to DynFlags up to the CoreToStg pass, which is almost the entire backend of GHC.


  • Jeffrey wrote a new HF proposal about writing a Haskell Optimization handbook and has started working on it