[Haskell-cafe] Error handling with safer-file-handling

[Florian Weimer]

How am I supposed to write an exception handle for an invocation
of System.IO.SaferFileHandles.openFile?

Currently, I have got this:

  case req of
    Open path -> do
      handle <-openFile (asAbsPath path) ReadMode
      liftIO $ forcePut result Success
      run handle

Follwing safer-file-handling-examples, I rewrote it to:

  case req of
    Open path -> do
      (do runRegionT $ do
            handle <-openFile (asAbsPath path) ReadMode
            liftIO $ forcePut result Success
            run handle)
      `catch` \(e :: IOException) -> do
        liftIO $ forcePut result $ Failure $ show e

(I would rather like to apply the handler to the openFile invocation
only, but the above matches the example code more closely.)

But this gives me a nice error message:

Handles.hs:48:17:
    Could not deduce (Control.Monad.CatchIO.MonadCatchIO
                        (RegionT s IO))
      from the context ()
      arising from a use of `catch' at Handles.hs:(48,17)-(54,50)
    Possible fix:
      add (Control.Monad.CatchIO.MonadCatchIO
             (RegionT s IO)) to the context of
        the polymorphic type `forall s. RegionT s pr α'
      or add an instance declaration for
         (Control.Monad.CatchIO.MonadCatchIO (RegionT s IO))
    In the expression:
          do { (do { runRegionT
                   $ do { handle <- openFile (asAbsPath path) ReadMode;
                          .... } }) }
        `catch`
          \ (e :: IOException)
              -> do { liftIO $ forcePut result $ Failure $ show e }
    In a case alternative:
        Open path
          -> do { (do { runRegionT $ do { ... } }) }
           `catch`
             \ (e :: IOException)
                 -> do { liftIO $ forcePut result $ Failure $ show e }
    In the expression:
        case req of {
          Open path
            -> do { (do { runRegionT $ ... }) }
             `catch`
               \ (e :: IOException) -> do { ... }
          Close -> return ()
          _ -> liftIO $ forcePut result $ Failure "handle not open" }

safer-file-handling-examples results in similar error messages, after
applying the s/runTopRegion/runRegionT/ fix.

This is with GHC 6.12.1, with ScopedTypeVariables enabled.