Fwd: Host-Oriented Template Haskell

[Ericson, John]

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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