Re: [Haskell-cafe] Intro to monad transformers

On 26 December 2010 19:00, michael rice wrote:

I lifted the code below from here:

http://en.wikibooks.org/wiki/Haskell/Monad_transformers

Since the wiki page doesn't say what needs to be imported, I'm guessing.

Not sure what is happening. Maybe someone can tell me.

instance Monad m => MonadPlus (MaybeT m) where     mzero     = MaybeT $ return Nothing     mplus x y = MaybeT $ do maybe_value <- runMaybeT x                             case maybe_value of                                  Nothing    -> runMaybeT y                                  Just value -> runMaybeT x

I haven't run the code but the definition of mplus here looks cyclic.