Question

I have three functions I map over, I wish to stop evaluation upon catching an exception.
I can catch the exception, but am not getting the behavior I want. It could be that I’m thinking about this problem in the incorrect way (maybe I shouldn’t be map a list of functions in this case), and would appreciate this being pointed out. Here’s what I think is the relevant code.

import qualified Control.Exception as C
data JobException = PreProcessFail
                  | JobFail
                  | ChartFail
                     deriving (Show, Typeable)

instance C.Exception JobException



type ProcessState = MVar ProcessConfig



data ProcessConfig = PConfig { model :: ServerModel
                             , ipAddress :: String
                             , cookie :: Cookie
                             } deriving Show

exceptionHandler :: JobException -> IO ()
exceptionHandler exception = do
   writeFile "testException.txt" ("caught exception " ++ (show exception))
-- much more functionality will be put here once I get the logic correct
preProcess :: ProcessState -> IO ()
preProcess sModel = do
   putStrLn ("preProcessing" )

initiateJob :: ProcessState -> IO ()
initiateJob sModel = do
   C.throw JobFail  
   putStrLn ("in progress")
makeChart :: ProcessState -> IO ()
makeChart sModel = do
     putStrLn ("chart making")

So now, when I test this out in ghci, this is what happens.

a <- mapM (flip Control.Exception.catch exceptionHandler) [preProcess world, initiateJob world, makeChart world]
Loading package filepath-1.2.0.0 ... linking ... done.
Loading package unix-2.4.2.0 ... linking ... done.
preProcessing
chart making

I should not be seeing the string “chart making”. How do I abort the evaluation of the list upon throwing an exception?

Answer 1

mapM maps the function then sequences the list. So you’ve got a catch around each action in the list separately. What you want is to sequence the list into a single action, and then catch the exception once so interrupting everything else in the list. The following works:

(flip Control.Exception.catch exceptionHandler) $ sequence_ [preProcess world, initiateJob world, makeChart world]