Add tryFindTopDir to look for the top dir without blowing up if it is

not found.

1 files changed, 17 insertions, 11 deletions

diff --git a/compiler/main/SysTools/BaseDir.hs b/compiler/main/SysTools/BaseDir.hs
index f67d2def6d..c4fc71b502 100644
--- a/compiler/main/SysTools/BaseDir.hs
+++ b/compiler/main/SysTools/BaseDir.hs
@@ -14,6 +14,7 @@
 module SysTools.BaseDir
   ( expandTopDir, expandToolDir
   , findTopDir, findToolDir
+  , tryFindTopDir
   ) where
 
 #include "HsVersions.h"
@@ -88,23 +89,28 @@ expandToolDir _ s = s
 -- | Returns a Unix-format path pointing to TopDir.
 findTopDir :: Maybe String -- Maybe TopDir path (without the '-B' prefix).
            -> IO String    -- TopDir (in Unix format '/' separated)
-findTopDir (Just minusb) = return (normalise minusb)
-findTopDir Nothing
+findTopDir m_minusb = do
+  maybe_exec_dir <- tryFindTopDir m_minusb
+  case maybe_exec_dir of
+      -- "Just" on Windows, "Nothing" on unix
+      Nothing -> throwGhcExceptionIO $
+          InstallationError "missing -B<dir> option"
+      Just dir -> return dir
+
+tryFindTopDir
+  :: Maybe String -- ^ Maybe TopDir path (without the '-B' prefix).
+  -> IO (Maybe String) -- ^ TopDir (in Unix format '/' separated)
+tryFindTopDir (Just minusb) = return $ Just $ normalise minusb
+tryFindTopDir Nothing
     = do -- The _GHC_TOP_DIR environment variable can be used to specify
          -- the top dir when the -B argument is not specified. It is not
          -- intended for use by users, it was added specifically for the
          -- purpose of running GHC within GHCi.
          maybe_env_top_dir <- lookupEnv "_GHC_TOP_DIR"
          case maybe_env_top_dir of
-             Just env_top_dir -> return env_top_dir
-             Nothing -> do
-                 -- Get directory of executable
-                 maybe_exec_dir <- getBaseDir
-                 case maybe_exec_dir of
-                     -- "Just" on Windows, "Nothing" on unix
-                     Nothing -> throwGhcExceptionIO $
-                         InstallationError "missing -B<dir> option"
-                     Just dir -> return dir
+             Just env_top_dir -> return $ Just env_top_dir
+             -- Try directory of executable
+             Nothing -> getBaseDir
 
 
 -- See Note [tooldir: How GHC finds mingw and perl on Windows]