I've been fighting with this silly bug: I have two functions using curl (Network.Curl) for logging in via POST and downloading pages via GET. They do the usual boring stuff; cookies go in a text file whose path is passed in to both functions. They work great individually. But, I'm calling them sequentially: cj <- make_cookie_jar li_result <- log_in cj username password html <- get_page (Just cj) my_article The symptom that I noticed was that the call to get_page wasn't using the cookies. I looked in the cookie file -- they're all there. And when I call get_page from ghci (with the same cookie file), it works. It turns out that between the calls to log_in and get_page, the cookie file is empty. The stupidest possible solution that works is, cj <- make_cookie_jar li_result <- log_in cj username password putStrLn "Waiting one second..." thread_sleep 1 html <- get_page (Just cj) my_article And this program is not at all time sensitive, so that actually works for me. But now I'm curious. Does anyone have an idea what sort of sorcery is going on? Here's the rest of log_in if it's useful. log_in :: FilePath -> String -> String -> IO Bool log_in cookie_jar username password = withCurlDo $ do -- Create a curl instance. curl <- initialize -- Perform the request, and get back a CurlResponse object. -- The cast is needed to specify how we would like our headers -- and body returned (Strings). resp <- do_curl_ curl login_url curl_opts :: IO CurlResponse -- Pull out the response code as a CurlCode. let code = respCurlCode resp case code of CurlOK -> return True error_code -> do hPutStrLn stderr ("HTTP Error: " ++ (show error_code)) -- If an error occurred, we want to dump as much information as -- possible. If this becomes a problem, we can use respGetInfo -- to query the response object for more information return False where post_submit :: String post_submit = submit_field ++ "=Log+In" post_username :: String post_username = username_field ++ "=" ++ username post_password :: String post_password = password_field ++ "=" ++ password post_data :: [String] post_data = [post_username, post_password, post_submit] post_opts :: [CurlOption] post_opts = [ CurlCookieSession True, CurlCookieJar cookie_jar, CurlPost True, CurlPostFields post_data ] curl_opts :: [CurlOption] curl_opts = default_curl_opts ++ post_opts default_curl_opts :: [CurlOption] default_curl_opts = [ -- The Global cache is not thread-friendly. CurlDNSUseGlobalCache False, -- And we don't want to use a DNS cache anyway. CurlDNSCacheTimeout 0, -- Follow redirects. CurlFollowLocation True, -- Give it a little time... CurlTimeout 45, -- For debugging. CurlVerbose True ]