Fwd: Host-Oriented Template Haskell

[Edward Z. Yang]

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 brown.edu> 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 mit.edu> wrote:
> >> Hello John
> >>
> >> You may find this implementation ticket of interest:
> >> https://ghc.haskell.org/trac/ghc/ticket/11378
> >>
> >> 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 brown.edu>
> >>> Date: Tue, Jan 19, 2016 at 1:17 PM
> >>> Subject: Host-Oriented Template Haskell
> >>> To: Haskell-Cafe <haskell-cafe at haskell.org>
> >>>
> >>>
> >>> 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]: https://github.com/ghcjs/ghcjs/blob/master/lib/ghcjs-prim/GHCJS/Prim/Internal/Build.hs
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