[Haskell-cafe] error vs. MonadError vs. fail

[Daniel McAllansmith]

On Tuesday 28 March 2006 07:29, Andrew Pimlott wrote:
> > MonadError is not up to this task as far as I can tell.
>
> Why not?  All that needs to be done is write the missing instances, eg
>
>     instance MonadError () Maybe where
>       throwError x           = Nothing
>       Nothing `catchError` f = f ()
>       Just x `catchError` f  = Just x
>
>     instance Error () where
>       noMsg                  = ()
>       strMsg s               = ()
>

How would you go about writing the Maybe based analogue of ErrorT?  What do 
you give to the handler in the instance of MonadError?


newtype ErrMaybeT e m a = ErrMaybeT { runErrMaybeT :: m (Maybe a) }

instance (Monad m, Error e) => Monad (ErrMaybeT e m) where
    return a = ErrMaybeT $ return (Just a)
    m >>= k  = ErrMaybeT $ do
        a <- runErrMaybeT m
        case a of
            Nothing -> return Nothing
            Just r -> runErrMaybeT (k r)
    fail msg = ErrMaybeT $ return Nothing

instance (Monad m, Error e) => MonadError e (ErrMaybeT e m) where
    throwError l     = ErrMaybeT $ return Nothing
    m `catchError` h = ErrMaybeT $ do
        a <- runErrMaybeT m
        case a of
            Nothing -> runErrMaybeT (h ???) --what to do here?
            Just r -> return (Just r)

f :: (MonadError String m) => Bool -> m Int
f b = if b
    then return 42
    else throwError "The boolean was false."

test1 b = do
    r <- runErrorT $ f b
    putStrLn (show r) --Left "..." or Right 42
    return ()

test2 b = do
    r <- runErrMaybeT $ f b
    putStrLn (show r) --Nothing or Just 42
    return ()


Daniel