Hello, I have the following code (fragment) I use to wrap execution of various processes in Haskell, in the spirit of Don Stewart's slides about "Scripting in Haskell". instance MonadExec IO where exec proc args = do (exit, out, err) <- liftIO $ readProcessWithExitCode proc args "" case exit of ExitFailure e -> throwError $ userError ("Fail execution of " ++ program proc args ++ ": " ++ (show e) ++", " ++err) ExitSuccess -> return out Here is the abridged declaration of MonadExec: class (Monad m) => MonadExec m where exec :: String -> [String] -> m String The issue I am facing is that readProcessWithExitCode does not seem to return the expected failures. When I run some failing process within windows command shell, I have: D:\projets\psug-dojo\lags>"d:/Program Files/scala-2.8.0/bin/scalac.bat" -d target\test-classes -classpath target\classes;D:\projets\ psug-dojo\lags\test-lib\specs_2.8.0-1.6.5.jar;D:\projets\psug-dojo\lags\test-lib\scalacheck_2.8.0-1.7.jar;D:\projets\psug-dojo\lags\ test-lib\junit-4.7.jar -d target\test-classes src\test\scala\oqube\lags\LagsTest.scala src\test\scala\oqube\lags\LagsTest.scala:86: error: not found: value beS_== bid.sublists(List(Request(1,2,3))) must beS_==(List(Nil,List(Request(1,2,3)))) ^ one error found D:\projets\psug-dojo\lags>echo %ERRORLEVEL% 1 D:\projets\psug-dojo\lags> but wrapping this same process in my "shell", I always got an ExitSuccess. What am I doing wrong ? Thanks Arnaud