[Haddock] [haddock] #256: Error when Template Haskell is used and GHC is dynamic

[haddock]

#256: Error when Template Haskell is used and GHC is dynamic
-------------------+--------------------------------------------------------
Reporter:  refold  |       Owner:     
    Type:  defect  |      Status:  new
Priority:  major   |   Milestone:     
 Version:  2.13.1  |    Keywords:     
-------------------+--------------------------------------------------------
 This was [https://github.com/haskell/cabal/issues/1266 initially reported
 as a Cabal bug], but I'm now convinced that it's better to solve this
 issue on the Haddock side.

 When trying to build documentation for a library that uses Template
 Haskell (for example, `traverse-with-class`) with GHC HEAD, I get the
 following error:

 {{{
 $ cabal configure -w ~/bin/ghc-head/bin/ghc
 Resolving dependencies...
 Configuring traverse-with-class-0.1...

 $ cabal haddock --with-haddock ~/bin/ghc-head/bin/haddock
 Running Haddock for traverse-with-class-0.1...
 Preprocessing library traverse-with-class-0.1...
 Haddock coverage:
    0% (  0 /  2) in 'Data.Generics.Traversable.Core'
  100% (  3 /  3) in 'Data.Generics.Traversable.TH'
 dist/build/tmp-5360/Data/Generics/Traversable/Instances.hs:1:1:
     cannot find normal object file
 ‛/tmp/.haddock-5369/Data/Generics/Traversable/Core.dyn_o’
     while linking an interpreted expression
 }}}

 However, everything works fine with GHC 7.6.

 The error is due to the fact that GHCi in GHC HEAD
 [http://ghc.haskell.org/trac/ghc/wiki/DynamicGhcPrograms uses the system
 linker] and so it requires the TH modules to be `.dyn_o` instead of `.o`
 (otherwise they can't be loaded because of the ABI mismatch). So the issue
 can be worked around by passing `--optghc=-dynamic-too` to `cabal
 haddock`:

 {{{
 $ cabal haddock --with-haddock ~/bin/ghc-head/bin/haddock --haddock-
 options="--optghc=-dynamic-too"
 Running Haddock for traverse-with-class-0.1...
 Preprocessing library traverse-with-class-0.1...
 Haddock coverage:
    0% (  0 /  2) in 'Data.Generics.Traversable.Core'
  100% (  3 /  3) in 'Data.Generics.Traversable.TH'
  100% (  1 /  1) in 'Data.Generics.Traversable.Instances'
   93% ( 13 / 14) in 'Data.Generics.Traversable'
 Documentation created: dist/doc/html/traverse-with-class/index.html
 }}}

 We can work around this in Cabal, but I think that the right place to fix
 this issue is Haddock itself (since it also happens when Haddock is run in
 standalone mode - i.e. not being driven by `cabal haddock`).

 To do this, you should do the equivalent of parsing the output of `ghc
 --info` and invoking GHC with the `-dynamic` option (or `-dynamic-too` if
 it's supported) when the output contains a `("GHC Dynamic","YES")` tuple.
 I'm not quite sure how to implement this using GHC API, but it should be
 possible.

-- 
Ticket URL: <http://trac.haskell.org/haddock/ticket/256>
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