Out Of Process Template Haskell

Luite Stegeman stegeman at gmail.com
Thu Dec 4 14:21:10 UTC 2014

On Thu, Dec 4, 2014 at 12:07 PM, Simon Peyton Jones <simonpj at microsoft.com>

> Luite's "out of process TH" work is amazing.  If someone had suggested it
> to me I'd have said "it sounds entirely impractical", but he showed that I
> would have been quite wrong.
I haven't had much time to actually help Moritz lately, due to other GHC
7.10 patches, the new GHCJS codegen and work to get Cabal support for GHCJS

Building/linking the code for the splices can be done in GHC without
additional dependencies, and with the recent changes in the
template-haskell package (if I haven't missed anything), serialization for
communication can also be implemented in a ligthtweight way based on
Generic or Data without additional dependencies.

I'd like to start simple and treat the Template Haskell runner as an
external build tool, possibly installable as a cabal package. GHC would
just start a runner process and communicate with it over the standard pipes
whenever it needs to run TH. The runner is then responsible for loading and
executing the code (in most cases a dynamic library for each splice) on the

I intend to start by making a patch that adds the runner to the 'settings'
file, such that users can enable TH in a stage1 (cross) compiler by
pointing GHC to the correct runner program. Other than the minor changes in
SysTools (for 'settings'), the code for this would be common to all
implementation alternatives.

We can then figure out whether this approach is flexible enough and further
work out configuration options and serialization protocol, or whether
Moritz' idea of loading a plugin to have more control over TH code
generation is worth the additional complexity (a different loading
mechanism would be required for plugins in cross compilers).

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