catching IO errors in a monad transformer stack

18 Jul 2013

      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

Eric Rasmussen

Alberto G. Corona

David Sorokin

Eric Rasmussen

Arie Peterson

Eric Rasmussen

John Lato

Eric Rasmussen

John Lato

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