[Haskell-cafe] Control.Monad.Error

Andrew Pimlott andrew at pimlott.net
Sun Jun 12 02:03:36 EDT 2005

I just discovered Control.Monad.Error, and all sorts of questions and
ideas came to mind.  I would be happy for any comments or pointers.

- Monad and MonadError are uncannily similar, especially if you ignore
  that ugly duckling fail:

    class (Monad m) => MonadError e m | m -> e where
        throwError :: e -> m a
        catchError :: m a -> (e -> m a) -> m a

  The instances for Either are exactly symmetrical.  Is there some more
  general way to look at this that makes the symmetry stand out?

- The instance MonadError IOError IO is problematic, because one might
  have preferred to use Control.Exception.Exception as the error type.
  I've been told that the latter is generally preferred, and it's what I
  usually want.  Any chance of the instance being changed?

- It's a shame that the utility functions in Control.Exception aren't
  defined in terms of MonadError.  Maybe they should at least be
  duplicated in Control.Monad.Error.

- Why not

    instance MonadError () Maybe where
      throwError x           = Nothing
      Nothing `catchError` f = f ()
      Just x `catchError` f  = Just x

    instance Error () where
      noMsg                  = ()
      strMsg s               = ()

  ?  This would seem to facilitate a variation of the "NotJustMaybe"
  pattern, using MonadError instead of Monad.  What I'm getting at is
  that it might be nice to see MonadError used more in preference to
  Monad.  Then we can get rid of fail. ;-)

- I found this function terribly useful:

    liftError :: (MonadError e m, MonadTrans t, MonadError e (t m)) =>
                    m a -> t m a
    liftError m = join (lift (liftM return m
                              (return . throwError)))

  However, I'm not sure exactly how to describe it (or what to call
  it--maybe liftTry).  It basically pulls errors from the inner monad to
  the outer monad.  One application is

    try :: MonadError e m => m a -> m (Either e a)
    try m = runErrorT (liftError m)

  The really nice thing for me is that I can use it with StateT to
  prevent errors from disturbing the state, as normally happens when you
  mix StateT with, say, IO:

    test1 = execStateT test 0 >>= print where
      test :: StateT Int IO ()
      test  = do  put 2
                  lift (fail "foo")
              `catchError` \e -> modify (+1)

  This prints 1, because the IOError obliterates all state changes in
  the first argument to catchError.  However,

    test2 = execStateT (runErrorT test) 0 >>= print where
      test :: ErrorT IOError (StateT Int IO) ()
      test  = do  put 2
                  liftError (fail "foo")
              `catchError` \e -> modify (+1)

  prints 3, because the IOError was caught by liftError, then passed
  down along with the state change.  Oh, and

    test2 = execStateT (runErrorT test) 0 >>= print where
      test :: ErrorT IOError (StateT Int IO) ()
      test  = do  put 2
                  lift (fail "foo")
              `catchError` \e -> modify (+1)

  raises an IOError, because the error is never seen by the outer monad
  (and catchError).


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