status of template haskell + cross compiling plans for 7.8?

Simon Marlow marlowsd at
Wed Jun 19 11:44:27 CEST 2013

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.


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] *On Behalf Of *Carter Schonwald
> *Sent:* 13 June 2013 22:44
> *To:* 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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