Detects when hsenv is activated and read the necessary GHC flags.

yesod/GhcBuild.hs

@@ -22,6 +22,7 @@ import qualified Control.Exception as Ex
 import           Control.Monad     (when)
 import           Data.IORef
 import           System.Process    (rawSystem)
+import           System.Environment (getEnvironment)
 
 import           CmdLineParser
 import           Data.Char         (toLower)
@@ -53,7 +54,8 @@ import           Util              (consIORef, looksLikeModuleName)
 getBuildFlags :: IO [Located String]
 getBuildFlags = do
   argv0 <- fmap read $ readFile "yesod-devel/ghcargs.txt" -- generated by yesod-ghc-wrapper
-  let (minusB_args, argv1) = partition ("-B" `isPrefixOf`) argv0
+  argv0' <- addHsenvArgs argv0
+  let (minusB_args, argv1) = partition ("-B" `isPrefixOf`) argv0'
       mbMinusB | null minusB_args = Nothing
                | otherwise = Just (drop 2 (last minusB_args))
   let argv1' = map (mkGeneralLocated "on the commandline") argv1
@@ -61,6 +63,14 @@ getBuildFlags = do
   (argv2, staticFlagWarnings) <- GHC.parseStaticFlags argv1'
   return argv2
 
+addHsenvArgs :: [String] -> IO [String]
+addHsenvArgs argv = do
+  env <- getEnvironment
+  return $ case (lookup "HSENV" env) of
+             Nothing -> argv
+             _       -> argv ++ hsenvArgv
+                 where hsenvArgv = words $ fromMaybe "" (lookup "PACKAGE_DB_FOR_GHC" env)
+
 buildPackage :: [Located String] -> FilePath -> FilePath -> IO Bool
 buildPackage a ld ar = buildPackage' a ld ar `Ex.catch` \e -> do
   putStrLn ("exception building package: " ++ show (e :: Ex.SomeException))