Hi! The exception handling is a difficult thing. It is usually simple enough but sometimes it can be very difficult, especially when using continuations within the monadic computation. To feel it, I often remember how the exceptions are handled in the F# async workflow (the sources are open), but their approach should be slightly adopted for Haskell what I did in one my simulation library (as far as I understand, the IO exception cannot arise in a pure value; therefore IOException should be caught in another place, namely in the liftIO function).
I'm not sure whether there is a common pattern for handling the exceptions (the mentioned MonadCatchIO instance contains a warning regarding ContT). Therefore it is reasonable to allow the programmer himself/herself to define these handlers through the type class. Thanks, David 19.07.2013, в 3:23, Alberto G. Corona написал(а): > 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. > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe
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