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

[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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