Hi Eric: The pattern may be the MonadCatchIO class:
http://hackage.haskell.org/package/MonadCatchIO-transformers 2013/7/18 Eric Rasmussen <ericrasmus...@gmail.com> > Hello, > > I am writing a small application that uses a monad transformer stack, and > I'm looking for advice on the best way to handle IO errors. Ideally I'd > like to be able to perform an action (such as readFile > "file_that_does_not_exist"), catch the IOError, and then convert it to a > string error in MonadError. Here's an example of what I'm doing now: > > {-# LANGUAGE FlexibleContexts #-} > > import Control.Monad.Error > import Control.Monad.State > > import System.IO.Error (tryIOError) > > catcher :: (MonadIO m, MonadError String m) => IO a -> m a > catcher action = do > result <- liftIO $ tryIOError action > case result of > Left e -> throwError (show e) > Right r -> return r > > This does work as expected, but I get the nagging feeling that I'm missing > an underlying pattern here. I have tried catch, catchError, and several > others, but (unless I misused them) they don't actually help here. The > tryIOError function from System.IO.Error is the most helpful, but I still > have to manually inspect the result to throwError or return to my > underlying monad. > > Since this has come up for me a few times now, I welcome any advice or > suggestions on alternative approaches or whether this functionality already > exists somewhere. > > Thanks! > Eric > > > > > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > > -- Alberto.
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