[Haskell-cafe] Re: Is my code too complicated?

Stephen Tetley wrote:

On 5 July 2010 10:39, Yves Parès wrote:

...

Then what is your alternative? How do you replace monad transformers?

Possibly more a case of doing without rather than replacing them with something else, you would amalgamate all the monadic effects you want into one monad.

E.g. State and Environment (reader) and partiality (Maybe)

newtype Amalgamated s e a = Amalgamated { getAmalgamated :: e -> s -> (Maybe a,st) }

instance Monad (Amalgamated s e) where return a = Amalgamated $ \e s -> return (Just a, st) m >>= k = Amalgamated $ \e s -- TODO (after the first coffee of the morning...)

That's what monad transformers are good for. Why reinvent the wheel? type Amalgamated s e m = MaybeT (StateT s (ReaderT e m)) This is all you need to create your own monad with the specified functionality. testComp :: Amalgamated Int Bool IO () testComp = do x <- return (Just 3) y <- ask z0 <- get when y $ sets_ (+1) z1 <- get inBase $ print (x, y, z0, z1) However, MaybeT as defined in the 'MaybeT' package will probably not work here. But this is not a transformer-related problem, just compatibility. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/