Provide --show-interface option to dump interfaces (#645)

* WIP: Provide --show-interface option to dump interfaces

Like ghcs own --show-iface this flag dumps a binary interface file to
stdout in a human (and machine) readable fashion. Currently it uses
json as output format.

* Fill all the jsonNull stubs

* Rework Bifunctor instance of DocH, update changelog and documentation

* replace changelog, bring DocMarkupH doc back

* Update CHANGES.md

* Update CHANGES.md

* Move Control.Arrow up

It would result in unused import if the Bifunctor instance is not generated.

1 files changed, 7 insertions, 0 deletions

diff --git a/haddock-api/src/Haddock.hs b/haddock-api/src/Haddock.hs
index 57ea5fea..554cb416 100644
--- a/haddock-api/src/Haddock.hs
+++ b/haddock-api/src/Haddock.hs
@@ -32,6 +32,7 @@ import Haddock.Backends.LaTeX
 import Haddock.Backends.Hoogle
 import Haddock.Backends.Hyperlinker
 import Haddock.Interface
+import Haddock.Interface.Json
 import Haddock.Parser
 import Haddock.Types
 import Haddock.Version
@@ -68,6 +69,7 @@ import System.Directory (doesDirectoryExist)
 import GHC hiding (verbosity)
 import Config
 import DynFlags hiding (projectVersion, verbosity)
+import ErrUtils
 import Packages
 import Panic (handleGhcException)
 import Module
@@ -164,6 +166,11 @@ haddockWithGhc ghc args = handleTopExceptions $ do
 
     dflags <- getDynFlags
 
+    forM_ (optShowInterfaceFile flags) $ \path -> liftIO $ do
+      mIfaceFile <- readInterfaceFiles freshNameCache [(("", Nothing), path)]
+      forM_ mIfaceFile $ \(_, ifaceFile) -> do
+        putMsg dflags (renderJson (jsonInterfaceFile ifaceFile))
+
     if not (null files) then do
       (packages, ifaces, homeLinks) <- readPackagesAndProcessModules flags files