[GHC] #7867: Allow template-haskell to communicate with itself between compilation units through the interface file

[GHC]

#7867: Allow template-haskell to communicate with itself between compilation units
through the interface file
--------------------------+------------------------------------------------
        Reporter:  errge  |            Owner:
            Type:         |           Status:  patch
  feature request         |        Milestone:  7.8.1
        Priority:         |          Version:  7.6.3
  normal                  |         Keywords:
       Component:         |     Architecture:  Unknown/Multiple
  Template Haskell        |       Difficulty:  Easy (less than 1 hour)
      Resolution:         |       Blocked By:  3725, 8337, 8340, 8397, 8398
Operating System:         |  Related Tickets:
  Unknown/Multiple        |
 Type of failure:         |
  None/Unknown            |
       Test Case:         |
        Blocking:         |
--------------------------+------------------------------------------------

Comment (by errge):

 Simon, first of all thanks for your reply.

 I'll do the wikipage, but in the meantime I'd like to clear up some
 confusion by answering your specific questions and giving further
 pointers.

 * What you miss is that `tcp_connect_timeout` is useful without being
 exported, because it can be used INSIDE  the TCP module.  Namely in the
 `connectToTCP :: Host -> Port -> IO ()` method.  That method can simply
 refer to the current flag value by `flags_tcp_connect_timeout`.  The user
 doesn't have to implement any passing of the value from Main through WGet
 to TCP, like in other getopt libraries. Please read these for further
 clarification:
     1. http://www.haskell.org/pipermail/haskell-
 cafe/2012-April/099641.html
     2. http://blog.risko.hu/2012/04/ann-hflags-0.html
     3.
 https://github.com/errge/hflags/blob/master/examples/package/test/main.hs,
 https://github.com/errge/hflags/blob/master/examples/package/TupMain.hs,
 https://github.com/errge/hflags/blob/master/examples/package/Tup.hs

 * You're right about qReifyModules, that's maybe the most shaky part of
 this.  I actually don't like the current way how this is handled inside
 GHC, but I'm quite sure that I don't see enough to judge this.  Currently
 GHC slurps up interface files and then uses them all while compiling
 different source files with --make.  This results in the ill-definedness
 you are talking about, but this is unfortunately already visible via the
 TH API, in #8408 I show that with a concrete example.  Let's discuss this
 in #8398, maybe we can come up with a better definition for module
 listing, I'll give it a try over the weekend to implement a better patch.

 I'm all in for implementing this feature slowly and cleanly for a future
 release if not for 7.8, right about that.  My end goal is to have the
 right and general compiler features, so the maintenance burden is minimal
 on me too regarding hflags and new GHC versions :)

 On the other hand, I'd really like to have a working hflags as soon as I
 can: by using dirty tricks internally inside hflags (like communicating
 via instances instead of annotations).  The only missing puzzle piece that
 I desperatly need support for from the compiler is to make a module orphan
 from TH.

 Currently I can do this by generating a fake orphan rule, but that would
 generate a warning for the user when compiling with -Wall, I'd like to
 have a warningless way.

 Can we maybe come up with a design/solution just for #8337 that is small
 and easy enough to be surely not messed up or regretted later?  I still
 prefer option 1, stating that module anntations make a module orphan seems
 pretty harmless, but useful from my point of view.  The patch would be
 also some clean and concise.

 I'll report back here when the more general summarizing wikipage that you
 asked for is ready and then we can continue to work on the more difficult
 changes for annotation reification and module listing in a more principled
 way.

 Can the already merged patches for #3725 and #8340 stay, please?

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7867#comment:13>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler