GHCJS now runs Template Haskell on node.js - Any interest in out of process TH for general cross compilation?

Simon Peyton Jones simonpj at
Thu Jul 3 16:18:33 UTC 2014


I lack the bandwidth to respond at any technical depth, but I’d like to make encouraging noises.  If you can figure out a way to make GHC do these things without making the compiler terribly complicated and making maintaining it harder, then I’m open to your proposals.

Several people seem to have said “oh yes, that’d be interesting”.


From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Luite Stegeman
Sent: 02 July 2014 17:14
To: ghc-devs; glasgow-haskell-users at
Subject: GHCJS now runs Template Haskell on node.js - Any interest in out of process TH for general cross compilation?

hi all,

I've added some code [1] [2] to GHCJS to make it run Template Haskell code on node.js, rather than using the GHC linker. GHCJS has supported TH for a long time now, but so far always relied on native (host) code for it. This is the main reason that GHCJS always builds native and JavaScript code for everything (another is that Cabal Setup.hs scripts need to be compiled to some host-runnable form, but that can also be JavaScript if you have node.js)

Now besides the compiler having to do twice the work, this has some other disadvantages:

- Our JavaScript code has the same dependencies (packages) as native code, which means packages like unix or Win32 show up somewhere, depending on the host environment. This also limits our options in choosing JS-specific packages.
- The Template Haskell code runs on the host environment, which might be slightly different from the target, for example in integer size or operating system specific constants.

Moreover, building native code made the GHCJS installation procedure more tricky, making end users think about libgmp or libiconv locations, since it basically required the same preparation as building GHC from source. This change will make installing much easier and more reliable (we still have to update the build scripts).

How it works is pretty simple:

- When any code needs to be run on the target (hscCompileCoreExpr, through the Hooks API new in GHC 7.8), GHCJS starts a node.js process with the thrunner.js [3] script,
- GHCJS sends its RTS and the Template Haskell server code [1] to node.js, the script starts a Haskell thread running the server,
- for every splice, GHCJS compiles it to JavaScript and links it using its incremental linking functionality. The code for the splice, including dependencies that have not yet been sent to the runner (for earlier splices), is then sent in a RunTH [4] message,
- the runner loads and runs the code in the Q monad, can send queries to GHCJS for reification,
- the runner sends back the result as a serialized Template Haskell AST (using GHC.Generics for the Binary instances).

All Template Haskell functionality is supported, including recent additions for reifying modules and annotations. I still need to clean up and push the patches for the directory and process packages, but after that, the TH code can read/write files, run processes and interact with them and make network connections, all through node.js.

Now since this approach is in no way specific to JavaScript, I was wondering if there's any interest in getting this functionality into GHC 7.10 for general cross compilation. The runner would be a native (target) program with dynamic libraries (or object files) being sent over to the target machine (or emulator) for the splices.

Thanks to Andras Slemmer from Prezi who helped build the initial proof of concept (without reification) at BudHac.



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