Re: [Haskell-cafe] catching IO errors in a monad transformer stack

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 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

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