Use bracket in `withCurrentDirectory`

Summary: There was a comment about that. Seems like a better behaviour.

Reviewers: austin

Reviewed By: austin

Subscribers: thomie, carter, simonmar

Differential Revision: https://phabricator.haskell.org/D419

utils/ghc-cabal/Main.hs

@@ -21,6 +21,7 @@ import Distribution.Verbosity
 import qualified Distribution.InstalledPackageInfo as Installed
 import qualified Distribution.Simple.PackageIndex as PackageIndex
 
+import Control.Exception (bracket)
 import Control.Monad
 import qualified Data.ByteString.Lazy.Char8 as BS
 import Data.List
@@ -70,14 +71,10 @@ die :: [String] -> IO a
 die errs = do mapM_ (hPutStrLn stderr) errs
               exitWith (ExitFailure 1)
 
--- XXX Should use bracket
 withCurrentDirectory :: FilePath -> IO a -> IO a
 withCurrentDirectory directory io
- = do curDirectory <- getCurrentDirectory
-      setCurrentDirectory directory
-      r <- io
-      setCurrentDirectory curDirectory
-      return r
+ = bracket (getCurrentDirectory) (setCurrentDirectory)
+           (const (setCurrentDirectory directory >> io))
 
 -- We need to use the autoconfUserHooks, as the packages that use
 -- configure can create a .buildinfo file, and we need any info that