The Monad type-class inherits from Applicative (from GHC 7.10 onward; see the Monad as applicative section for more on this):
class (Applicative m) => Monad m where return :: a -> m a (>>=) :: m a -> (a -> m b) -> m b
The return function looks just like the pure function of the Applicative type-class (it lifts a value to Monad).
The bind operator (>>=) combines a Monad (m a) with a function (a -> m b), which we'll call a monadic
function. The monadic function acts on type a of the first monad and returns a new monad of type (m b).
Let's make our Maybe' type a Monad:
import Control.Monad import Control.Applicative data Maybe' a = Just' a | Nothing' deriving (Show) instance Functor Maybe' where –- ... instance Applicative Maybe' where –- ... instance Monad Maybe' where return x = Just' x Nothing' >>= _ = Nothing' (Just' x) >>= f = (f x)
The bind operator for Maybe' says:
Given
Nothing', ignore the monadic function and simply return...