Sorry for not being clear. That's just the (abbreviated) type signature of >>=, which is formally written as (>>=) :: Monad m => m a -> (a -> m b) -> m b

Sorry for not being clear. That's just the (abbreviated) type signature of >>=, which is formally written as (>>=) :: Monad m => m a -> (a -> m b) -> m b It's not the definition of >>=, though. The definition is left up to the specific Monad instance that's defining it (which is basically the larger point that I was trying to make). In English, that type signature basically says that >>= is a function which takes a monad and a function that operates on the monad's "contents" as its arguments, and returns the result of applying the function to the monad's contents. Any data type for which that abstract structure makes sense (and that also complies with the basic "monad laws") can be made an instance of Monad and provide its own definition for how >>= operates. That definition can be wildly different for different data types; the specific definition that the article focuses on is the one for the Maybe type.