Am 09.12.2012 um 00:27 schrieb Holger Siegel: > For deriving a monoid instance of w from monad (Writer w), you will need > function execWriter:: Writer w a -> w, but in case of a general instance of > (MonadWriter w m) you would have to use function listen :: m a -> m (a, w) > that will only provide you a value of type (m w), but not of type w.
sorry, I meant function execWriterT :: Monad m => WriterT w m a -> m w. > Therefore, > I'm not yet convinced that every instance of (MonadWriter w m) gives rise > to a monoid instance of w. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
