Re: [Haskell-cafe] Newbie Question about Error Handling

25 Jul 2005


      ...
We're being clever and returning from IO not a Char, but a MyMonad Char
(mimp).  Then, we run that, which either produces the answer we wanted,
or throws an error in MyMonad.
Thanks. That did the trick! :-)
Regards
...
However, it's still rather cumbersome, so here's a function similar to
liftError that works for your monad.
-- I'll assume you have such a function
    fromIOError :: IOError -> MyErrorType
-- The name is intended to convey that IO errors are caught and
    -- reintroduced into MyMonad.  Better suggestions welcome.
    liftIOTrap :: IO a -> MyMonad a
    liftIOTrap io = do mx <- liftIO (do x <- io
                                        return (return x)
                                     `catchError`
                                     (\e -> return (throwError
                                                      (fromIOError e))))
                       mx
foo :: MyMonad a
    foo = do
                inp <- liftIOTrap (getChar
                                    `catchError`
                                    (\e -> if isEOFError e then return 
'\0'
                                                           else throwError 
e))
                ...
Andrew
[1] http://haskell.org/pipermail/haskell-cafe/2005-June/010361.html
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Re: [Haskell-cafe] Newbie Question about Error Handling

Gerd M