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

Carter Schonwald carter.schonwald at
Thu Jul 3 00:54:20 UTC 2014

This would probably be a great boon for those trying to use haskell for
Android and IOS right? how might the emulation setup work for those?

On Wed, Jul 2, 2014 at 2:20 PM, Carter Schonwald <carter.schonwald at
> wrote:

> 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]
>> _______________________________________________
>> Glasgow-haskell-users mailing list
>> Glasgow-haskell-users at
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