Require -fexternal-interpreter support for future TH changes?

[Edward Z. Yang]

Hello Simon,

I have no exception to having it be default and dropping the special
case support for building profiled/dynamic so that TH works.  But
I don't think support for loading code in-process for GHC should be dropped,
c.f. Manuel's email
https://mail.haskell.org/pipermail/ghc-devs/2015-November/010491.html
and also the necessity to run code in-process for typechecking plugins,
etc.

Edward

Excerpts from Simon Marlow's message of 2016-06-22 04:51:12 -0400:
> *Background*
> 
> A few months ago I added -fexternal-interpreter to GHC:
> 
>    - docs:
>    http://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html#ghc-flag--fexternal-interpreter
>    - wiki, rationale: https://ghc.haskell.org/trac/ghc/wiki/RemoteGHCi
> 
> When -fexternal-interpreter is used, GHC runs interpreted code in a
> separate subprocess, and communicates with it using binary messages over a
> pipe.
> 
> -fexternal-interpreter currently implements all of TH, quasi-quoting,
> annotations, and all the GHCi features except for some features of the
> debugger.  It is also now implemented on Windows, thanks to Tamar Christina.
> 
> *Proposal*
> 
> I'd like to propose that going forward we commit to maintaining full
> support for -fexternal-interpreter, with a view to making it the default.
> 
> Why?
> 
>    - -fexternal-interpreter will be a prerequisite for GHCJS support, so
>    maintaining full support for TH in -fexternal-interpreter will ensure that
>    everything that works with GHC works with GHCJS.
>    - We will be able to make simplifications in GHC and the build system
>    once -fexternal-interpreter is the default, because when compiling with
>    -prof or -dynamic we won't have to compile things twice any more.
>    - Ultimately we don't want to have two ways of doing everything, because
>    that's harder to maintain.
> 
> How?
> 
>    - I'll make all the TH and quasi-quoting tests run with and without
>    -fexternal-interpreter, so it will break validate if one of these fails.
> 
> *Why now?*
> 
> There are some TH changes in the pipeline that will need special attention
> to work with -fexternal-interpreter.  e.g.
> https://phabricator.haskell.org/D2286 and
> https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/Introspective, so I'd
> like to raise it now so we can keep the issue in mind.
> 
> 
> Cheers
> 
> Simon