It is probably a bad idea in #Haskell,
but still surprising: #ghc does not
complain about using "do" to define (>>=) itself.
It seams to cause and endless loop.

newtype BadIdea a = B a
        deriving Show

instance Functor BadIdea where
        fmap f (B a) = B (f a)

instance Applicative BadIdea where
        pure x = B x
        (B f) <*> B x = B (f x)

instance Monad BadIdea where
        return x = B x
        ma >>= f = do
                 a <- ma
                 f a
-- instead of
--        (B a) >>= f = f a

main = print
        (B 5 >>= (\x -> B (2*x)))