[Haskell-cafe] Unexpected results from ExitCode

[Dean Herington]

At 8:45 PM -0700 11/3/10, Peter Schmitz wrote:
>I have a program (test.hs):
>
>>  module Main (main) where
>>  import System.Exit
>>  main :: IO ExitCode
>>  main = do
>>     return (ExitFailure 1)
>
>
>In another program, I invoke it via 'system':
>
>>     exitCode <- system ".\\test.exe"
>>     case (exitCode) of
>>        ExitFailure failCnt -> do
>>           putStrLn $ "-- Fail count is: " ++ show failCnt
>>           exitFailure
>>        ExitSuccess -> do
>>           putStrLn $ "-- OK."
>>           exitSuccess
>
>
>but it always gets ExitSuccess (not ExitFailure failCnt as I expected).
>(I am running under Windows.)
>
>My other use of system (such as with ghc commands) works okay,
>sometimes getting success, sometimes failure, as expected.
>
>Any suggestions much appreciated.
>Thanks,
>-- Peter

The value returned by `main` is always discarded.  You need to use `exitWith`:

main = exitWith (ExitFailure 1)