[GHC] #9010: TemplateHaskell leads to an "unknown symbol" error

[GHC]

#9010: TemplateHaskell leads to an "unknown symbol" error
-------------------------------------+-------------------------------------
        Reporter:  Feuerbach         |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.6.3
      Resolution:  fixed             |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Changes (by rwbarton):

 * status:  infoneeded => closed
 * priority:  high => normal
 * version:  7.10.2 => 7.6.3
 * resolution:   => fixed


Comment:

 Okay, that's helpful and different from how my package was built. I
 managed to reproduce the error you saw by building an archive with an
 undefined symbol reference and specifying it with `extra-libraries:` and
 `extra-lib-dirs:` in `hs-opencv-binding.cabal`. (Of course, it's not a bug
 that GHC fails when there really is an undefined symbol!)

 It turns out that the GHC runtime linker is still used in the following
 situation:

 When loading a package into GHCi, the last step is to load libraries
 specified in the `extra-libraries:` field of the package registration. For
 each such library, GHCi will first try to find a matching shared library
 in the library path, then try to find a matching static library in the
 library path, then ask gcc where to find a shared library, and if all
 those fail, assume that `dlopen` will know where to find the shared
 library. (`compiler/ghci/Linker.hs`, function `locateLib`.) It is that
 second case that you are apparently in (I guess you only built OpenCV as a
 static library) and in that case the static library is loaded by the GHC
 runtime linker (in `linkPackage` in the same file).

 That is all fine, except that the GHC runtime linker apparently doesn't
 understand this `"u"` symbol type. But that is almost certainly unrelated
 to the original ticket here, so I reset the ticket status back to its
 previous state.

 I recommend that you simply use dynamic OpenCV libraries, if at all
 possible. If not, you will apparently need support for this `"u"` symbol
 type in the GHCi runtime linker. I'm not sure I understand exactly what
 the semantics of this symbol type are supposed to be, but it looks fairly
 straightforward to implement. However, that would obviously require a
 patch to GHC... so it would probably be easier for you just to use dynamic
 OpenCV libraries, and avoid the hairiness of the GHC runtime linker.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9010#comment:18>
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