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

Carter Schonwald carter.schonwald at
Wed Jul 2 18:20:53 UTC 2014

wow, this is great work!

If theres a clear path to getting the generic tooling into 7.10, i'm all
for it :) (and willing to help on concrete mechanical subtasks)

On Wed, Jul 2, 2014 at 12:14 PM, Luite Stegeman <stegeman at> wrote:

> 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.
> cheers,
> Luite
> [1]
> [2]
> [3]
> [4]
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