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

15 Nov 2010

      On Sun, Nov 14, 2010 at 12:38 PM, Florian Weimer  wrote:
...
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.)
You should be able to apply 'catch' directly to the openFile invocation.

Do note that a RegionT doesn't have an instance for MonadCatchIO
anymore. In regions-0.8[1] I switched to using MonadPeelIO from
monad-peel[2] instead. So import 'catch' from Control.Exception.Peel.
...
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.
Thanks for reporting this!

I forgot to update the safer-file-handles-examples[3]. They should be fine now.

Regards,

Bas

[1] http://hackage.haskell.org/package/regions-0.8
[2] http://hackage.haskell.org/package/monad-peel-0.1
[3] darcs get http://code.haskell.org/~basvandijk/code/safer-file-handles-examples

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

Bas van Dijk