Add a way to find out if a given Compiler supports parallel --make.

Cabal/Distribution/Simple/Compiler.hs

@@ -69,7 +69,8 @@ module Distribution.Simple.Compiler (
         languageToFlags,
         unsupportedLanguages,
         extensionsToFlags,
-        unsupportedExtensions
+        unsupportedExtensions,
+        parmakeSupported
   ) where
 
 import Distribution.Compiler
@@ -79,7 +80,7 @@ import Language.Haskell.Extension (Language(Haskell98), Extension)
 
 import Control.Monad (liftM)
 import Data.List (nub)
-import qualified Data.Map as M (Map)
+import qualified Data.Map as M (Map, lookup)
 import Data.Maybe (catMaybes, isNothing)
 import System.Directory (canonicalizePath)
 
@@ -214,3 +215,12 @@ extensionsToFlags comp = nub . filter (not . null)
 
 extensionToFlag :: Compiler -> Extension -> Maybe Flag
 extensionToFlag comp ext = lookup ext (compilerExtensions comp)
+
+-- | Does this compiler support parallel --make mode?
+parmakeSupported :: Compiler -> Bool
+parmakeSupported comp =
+  case compilerFlavor comp of
+    GHC -> case M.lookup "Support parallel --make" (compilerProperties comp) of
+      Just "YES" -> True
+      _          -> False
+    _   -> False

Cabal/Distribution/Simple/Program/GHC.hs

@@ -25,7 +25,6 @@ import Distribution.Version
 import Language.Haskell.Extension   ( Language(..), Extension(..) )
 
 import Data.Monoid
-import qualified Data.Map as M      ( lookup )
 
 -- | A structured set of GHC options/flags
 --
@@ -265,11 +264,11 @@ renderGhcOptions comp opts
   , [ "-split-objs" | flagBool ghcOptSplitObjs ]
 
 
-  , let numJobs = fromFlagOrDefault 1 (ghcOptNumJobs opts) in
-    case M.lookup "Support parallel --make" (compilerProperties comp) of
-    Just "YES" ->
-      if numJobs > 1 then ["-j" ++ show numJobs] else []
-    _          -> []
+  , if parmakeSupported comp
+    then
+      let numJobs = fromFlagOrDefault 1 (ghcOptNumJobs opts)
+      in if numJobs > 1 then ["-j" ++ show numJobs] else []
+    else []
 
   --------------------
   -- Dynamic linking