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GHC DevX May 2022 Update

· 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.