m a: means a type which take a parameter (another type).

For example,

data Maybe a = Nothing | Just a

    - The m is for "Maybe"
    - The a is for "a"

For example:

    (>>=) (Just 3) (\ x -> Just (show x))

should returns: Just "3".

    (>>=) Nothing (\x -> Just (show x))

better written

    Nothing >>= (\x -> Just (show x))

should returns: Nothing