Jeremy Shaw wrote: : | However, I think this is buggy, because changes | to 's' and 'w' will be lost if 'm' raises an | exception. : That's determined by the way you stack your monad transformers when declaring the type: adding error handling to a writer monad, or adding writing to an error handling monad. For a concrete example, see the result types in the following. The first has the Either inside the tuple, and the second has the tuple inside the Either. Prelude> :t Control.Monad.Writer.runWriter . Control.Monad.Error.runErrorT Control.Monad.Writer.runWriter . Control.Monad.Error.runErrorT :: Control.Monad.Error.ErrorT e (Control.Monad.Writer.Writer w) a -> (Either e a, w) Prelude> :t either Left Right . Control.Monad.Writer.runWriterT either Left Right . Control.Monad.Writer.runWriterT :: Control.Monad.Writer.WriterT w (Either a) a1 -> Either a (a1, w) Regards, Tom