status of template haskell + cross compiling plans for 7.8?

CJ van den Berg cj at
Fri Jun 21 14:57:04 CEST 2013

Template Haskell support in ghc-android is not complete.

ghc-android merely builds a stage1 cross-compiler with template
Haskell enabled. It still needs work done to allow the stage1
cross-compiler to load host architecture libraries from the stage0
host compiler’s library path. I’m hoping that it shouldn’t be too
difficult, but I haven’t had a chance to work on it yet.

This technique of course requires that the user install the required
libraries for both compilers, but I don’t see that as a problem. A
nice side effect of doing it this way is that you can have multiple
cross-compilers relying on the same stage0 host compiler. This is
great for multiple arch targets like Android.

On 2013-06-21 13:20, Alp Mestanogullari wrote:
> I recently came across this:
> The "build" script there together with the patches in the 'patch'
> directory seem to take care of the issues mentioned on this thread. Of
> course, that's quite a hacky way to make this work, but maybe Luite's
> work and this one could inspire a good long-term solution.
> On Wed, Jun 19, 2013 at 11:44 AM, Simon Marlow <marlowsd at
> <mailto:marlowsd at>> wrote:
>     The issue that we hit sooner than this is that when we build a
>     cross-compiler, we build all the libs for the target platform and
>     not the host platform (see
>     <>__).  So
>     to make TH work, we have to change the way we build cross compilers,
>     and make the build system able to build and install two sets of
>     libs.  This is probably a big job.
>     I think it might be possible to avoid needing a multi-target GHC, as
>     long as you have two identical GHCs, one that produces code for the
>     host platform (host-GHC) and one for the target platform
>     (cross-GHC).  You compile all the code once with host-GHC, and then
>     again with the cross-GHC, the second time using the .o files
>     produced by host-GHC to run the TH code.
>     Cheers,
>             Simon
>     On 17/06/13 14:52, Simon Peyton-Jones wrote:
>         I have not been following the details of this debate, but there is a
>         good reason why TH doesn’t work on a cross compiler. Specifically,
>         suppose module M imports module X, which defines a function mkD
>         that M
>         calls in a splice, thus $(mkD “wobble”).
>         Currently, we compile X to X.o, and when compiling M we
>         dynamically link
>         X.o (and all the other libraries it relies on) into GHC so that
>         we can
>         call foo.  Obviously X.o has to be runnable on the machine doing the
>         compiling, so if X.o is for some other architecture that’s not
>         going to
>         work.
>         There is no reason in principle why one could not compile X into
>         X.bytecode (along with all its dependencies), where X.bytecode is
>         architecture independent. Then X.bytecode could be interpreted
>         on any
>         platform.  But GHC has no mechanism for doing this at all.  
>         I’ve always
>         take the view that if you want X.bytecode, you may as well load
>         X.hs and
>         translate it into bytecode.  A bit slower, to be sure, but maybe
>         fast
>         enough.  But don’t forget those libraries.
>         Anyway that’s the state of play.   Have fun!
>         Simon
>         *From:*ghc-devs-bounces at
>         <mailto:ghc-devs-bounces at>
>         [mailto:ghc-devs-bounces at
>         <mailto:ghc-devs-bounces at>] *On Behalf Of *Carter
>         Schonwald
>         *Sent:* 13 June 2013 22:44
>         *To:* ghc-devs at <mailto:ghc-devs at>
>         *Subject:* status of template haskell + cross compiling plans
>         for 7.8?
>         Hey All,
>         Whats the story planned for template haskell + cross compiler
>         support
>         come ghc 7.8?
>         I understand theres a lot of Template Haskell design underway,
>         some of
>         which will help support tools like Manuel's Inline-Objective-C work.
>         Does this mean that factored out within this reorganization is a
>         better
>         story for cross compilation?
>         Especially since one kill "app" for the Inline-Objective-C template
>         haskell work would be writing IOS applications. Though I guess
>         that also
>         touches on the need to sort out supporting "FAT" ARM binaries
>         too, right?
>         This intersects with a few different large subsets of tickets,
>         so i'm
>         not sure if any single ticket is the right fora for this question.
>         thanks!
>         -Carter
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> -- 
> Alp Mestanogullari
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CJ van den Berg

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