Host-Oriented Template Haskell

Moritz Angermann moritz at
Thu Jan 21 06:55:36 UTC 2016

Colour me an interested party!  For iOS one currently builds two
different ghc compiler.  One for the simulator (i386) and one for the
actual device (arm).  My initial work on bringing ghcjs’s TH support,
(which basically ships the TH out to a process on the target to compile
and brings back the compiled TH into the cross compiler) to ghc, was
based on the idea of having a TH handler plugin, which would handle the
TH parts, and could then be adapted for different TH situations.  This
of course could only work if the plugin was compiled with the same version
of ghc, that the cross compiler was.

I wonder if the multi-target pipeline might be simpler to achieve with
the llvm backend.

If someone is taking a shot at this, I’d like to help!


> On Jan 21, 2016, at 2:44 PM, Edward Z. Yang <ezyang at> wrote:
> Yes, in principle, a compiler that supports cross-compiling to many
> targets without needing to be configured (at build time) which
> target it should support is ideal.  Unfortunately, currently GHC
> relies on autoconf to determine all of the necessary parameters
> for a cross-compile.  I am not sure how feasible it would be to
> allow these parameters to be configured at runtime: it might be
> easy, it might be hard!  (And of course you have to arrange
> to have cross-compiled all the boot libraries too...)
> In any case, I think that a GHC that permits changing cross-compilation
> targets at runtime is a fine goal, and if it is supported, then we
> would only have to make adjustments to GHC internally to distinguish
> between the interfaces/objects for various targets (including itself.)
> Edward
> Excerpts from Ericson, John's message of 2016-01-20 21:23:28 -0800:
>> Oops, Sent just to Edward by mistake.
>> ----
>> That all sounds good to me---I think what I was thinking all along in
>> fact, unless I am still missing some nuance :).
>> My idea of using a multi-target compiler was to avoid needing stage0
>> to be the same version as stage1 (or adding a new stage, stage1 as in
>> your alternative). The cross compiler is multi-target so it doesn't
>> need to link another compiler to load plugins / run "host-oriented
>> Template Haskell". I mentioned the usecase of trying to build a stage2
>> which needs plugins / Host-Oriented Template Haskell, but it could
>> well apply to building anything else with the need for those to things
>> (and not normal TH, which cannot be built with a cross compiler)
>> In diagrams, rather than:
>>> stage0 (version foo, platform-x -> platform-x)
>>> ==> stage1 (version foo, platform-x -> platform-y)
>>> ==> some program (with plugins, with host-oriented TH, without normal TH)
>> or
>>> stage0 (version bar, platform-x -> platform-x)
>>> ==> stage1 (version foo, platform-x -> platform-x) ->
>>> ==> stage2 (version foo, platform-x -> platform-y) ->
>>> ==> some program (with plugins, with host-oriented TH, without normal TH)
>> do just
>>> stage0 (version bar, platform-x -> platform-x)
>>> ==> stage1 (version foo, platform-x -> {platform-x, platform-y})
>>> ==> some program (with plugins, with host-oriented TH, without normal TH)
>> Now it may be that building such a multi-target compiler today is just
>> as arduous as making sure one has a native compiler of the same
>> version before building the cross compiler, so my idea is of no
>> practical value. But if so, I hope in the long term that could be
>> fixed.
>> John
>> On Wed, Jan 20, 2016 at 8:37 PM, Edward Z. Yang <ezyang at> wrote:
>>> Hello John,
>>> Almost!
>>> As you point out, if you just want a cross-compiler, you don't
>>> need to build stage2.  The real problem is that we want stage0
>>> to be *the same version* as stage1, so that stage0 has the correct
>>> API in order to load stage0 libraries which are going to be used
>>> for TH, plugins, etc.  (Alternately, build a non-cross-compiler
>>> stage1, and then a cross-compiler stage2--NOT a cross compiled
>>> stage 2.)
>>> Edward
>>> Excerpts from Ericson, John's message of 2016-01-19 16:13:24 -0800:
>>>> From #11378:
>>>>> Can we actually link against the old version of ghc? Yes we can! All we need is
>>>>> (1) for it to have been built with a different IPID, and
>>>>> (2) to use module renaming to rename the relevant modules to a different
>>>>> module name, so that we can import them without a conflict.
>>>> If I understand this, you are saying stage1 links stage0 (the normal,
>>>> non-cross-compiler used to build stage1), to build stage2? I agree
>>>> that should work, but if stage1 is multi-target isn't that level of
>>>> complexity not even needed? I'm not too familiar with GHC or its build
>>>> system, but a multi-target compiler that doesn't support normal TH
>>>> doesn't seem that hard to build in principle.
>>>> John
>>>> On Tue, Jan 19, 2016 at 3:59 PM, Ericson, John <john_ericson at> wrote:
>>>>> Ah thanks for the link. Template Haskell with only {-# TH-ONLY #-}
>>>>> imports is precisely equivalent to what I propose. I find a clean
>>>>> separation with that and normal TH useful, however, precisely so the
>>>>> stage2 compiler's source can include such a thing.
>>>>> On Tue, Jan 19, 2016 at 3:20 PM, Edward Z. Yang <ezyang at> wrote:
>>>>>> Hello John
>>>>>> You may find this implementation ticket of interest:
>>>>>> Edward
>>>>>> Excerpts from Ericson, John's message of 2016-01-19 15:15:15 -0800:
>>>>>>> Cross-posting this as was suggested on the Haskell-Cafe list. While I
>>>>>>> envision this as normal feature that anyone can use, in practice its
>>>>>>> biggest consumers would be GHC devs.
>>>>>>> John
>>>>>>> ---------- Forwarded message ----------
>>>>>>> From: Ericson, John <john_ericson at>
>>>>>>> Date: Tue, Jan 19, 2016 at 1:17 PM
>>>>>>> Subject: Host-Oriented Template Haskell
>>>>>>> To: Haskell-Cafe <haskell-cafe at>
>>>>>>> As is well known, TH and cross-compiling do not get along. There are
>>>>>>> various proposals on how to make this interaction less annoying, and I
>>>>>>> am not against them. But as I see it, the problem is largely inherent
>>>>>>> to the design of TH itself: since values can (usually) be lifted from
>>>>>>> compile-time to run-time, and normal definitions from upstream modules
>>>>>>> to downstream modules' TH, TH and normal code must "live in the same
>>>>>>> world".
>>>>>>> Now this restriction in turn bequeaths TH with much expressive power,
>>>>>>> and I wouldn't advocate getting rid of it. But many tasks do not need
>>>>>>> it, and in some cases, say in bootstrapping compilers[1] themselves,
>>>>>>> it is impossible to use TH because of it, even were all the current
>>>>>>> proposals implemented.
>>>>>>> For these reason, I propose a new TH variant which has much harsher
>>>>>>> phase separation. Normal definitions from upstream modules can not be
>>>>>>> used, lifting values is either not permitted or is allowed to fail
>>>>>>> (because of missing/incompatible definitions), and IO is defined to
>>>>>>> match the behavior of the host, not target, platform (in the cross
>>>>>>> compiling case). The only interaction between the two phases is that
>>>>>>> quoted syntax is resolved against the the run-time phase's definitions
>>>>>>> (just like today).
>>>>>>> Some of you may find this a shoddy substitute for defining a subset of
>>>>>>> Haskell which behaves identically on all platforms, and optionally
>>>>>>> constraining TH to it. But the big feature that my proposal offers and
>>>>>>> that one doesn't is to be able to independently specify compile-time
>>>>>>> dependencies for the host-oriented TH---this is analogous to the
>>>>>>> newish `Setup.hs` dependencies. That in turns leads to what I think is
>>>>>>> the "killer app" for Host-Oriented TH: exposing the various
>>>>>>> prepossessors we use (alex, happy, hsc2hs, even CPP) into libraries,
>>>>>>> and side-stepping any need for "executable dependencies" in Cabal.
>>>>>>> Note also that at least hsc2hs additionally requires host-IO---there
>>>>>>> may not even exist a C compiler on the target platform at all.
>>>>>>> Finally, forgive me if this has been brought up before. I've been
>>>>>>> thinking about this a while, and did a final pass over the GHC wiki to
>>>>>>> make sure it wasn't already proposed, but I could have missed
>>>>>>> something (this is also my first post to the list).
>>>>>>> John
>>>>>>> [1]:
>>>>>> _______________________________________________
>>>>>> ghc-devs mailing list
>>>>>> ghc-devs at
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Moritz Angermann
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