Add -j[n] CLI param to Haddock executable

It translates to `--ghcopt=-j[n]`

2 files changed, 13 insertions, 3 deletions

diff --git a/haddock-api/src/Haddock.hs b/haddock-api/src/Haddock.hs
index 3543d8e2..8bf932df 100644
--- a/haddock-api/src/Haddock.hs
+++ b/haddock-api/src/Haddock.hs
@@ -151,12 +151,17 @@ haddockWithGhc ghc args = handleTopExceptions $ do
   sinceQual <- rightOrThrowE (sinceQualification flags)
 
   -- inject dynamic-too into flags before we proceed
-  flags' <- ghc flags $ do
+  flags'' <- ghc flags $ do
         df <- getDynFlags
         case lookup "GHC Dynamic" (compilerInfo df) of
           Just "YES" -> return $ Flag_OptGhc "-dynamic-too" : flags
           _ -> return flags
 
+  flags' <- pure $ case optParCount flags'' of
+    Nothing       -> flags''
+    Just Nothing  -> Flag_OptGhc "-j" : flags''
+    Just (Just n) -> Flag_OptGhc ("-j" ++ show n) : flags''
+
   -- bypass the interface version check
   let noChecks = Flag_BypassInterfaceVersonCheck `elem` flags
 
diff --git a/haddock-api/src/Haddock/Options.hs b/haddock-api/src/Haddock/Options.hs
index eda40935..65aacc61 100644
--- a/haddock-api/src/Haddock/Options.hs
+++ b/haddock-api/src/Haddock/Options.hs
@@ -24,6 +24,7 @@ module Haddock.Options (
   optSourceCssFile,
   sourceUrls,
   wikiUrls,
+  optParCount,
   optDumpInterfaceFile,
   optShowInterfaceFile,
   optLaTeXStyle,
@@ -110,6 +111,7 @@ data Flag
   | Flag_PackageVersion String
   | Flag_Reexport String
   | Flag_SinceQualification String
+  | Flag_ParCount (Maybe Int)
   deriving (Eq, Show)
 
 
@@ -221,7 +223,9 @@ options backwardsCompat =
     Option [] ["package-version"] (ReqArg Flag_PackageVersion "VERSION")
       "version of the package being documented in usual x.y.z.w format",
     Option []  ["since-qual"] (ReqArg Flag_SinceQualification "QUAL")
-      "package qualification of @since, one of\n'always' (default) or 'only-external'"
+      "package qualification of @since, one of\n'always' (default) or 'only-external'",
+    Option ['j'] [] (OptArg (\count -> Flag_ParCount (fmap read count)) "n")
+      "load modules in parallel"
   ]
 
 
@@ -304,10 +308,11 @@ optShowInterfaceFile flags = optLast [ str | Flag_ShowInterface str <- flags ]
 optLaTeXStyle :: [Flag] -> Maybe String
 optLaTeXStyle flags = optLast [ str | Flag_LaTeXStyle str <- flags ]
 
-
 optMathjax :: [Flag] -> Maybe String
 optMathjax flags = optLast [ str | Flag_Mathjax str <- flags ]
 
+optParCount :: [Flag] -> Maybe (Maybe Int)
+optParCount flags = optLast [ n | Flag_ParCount n <- flags ]
 
 qualification :: [Flag] -> Either String QualOption
 qualification flags =