Higher-order kinds

Types classify values at different levels of abstraction, for example:

­­-- Primitive types 
­­-- "a string"   :: String 
­­-- 12             :: Int  
 
--­­ instances of higher­order, parametrized types 
­­-- Just 10      :: Maybe Int 
­­-- Left 10      :: Either Int b 
 
­­-- functions are first class values 
­­-- (* 2)          :: Num a => a ­> a 
 
­­-- type­constructors are functions 
­­-- Just          :: a ­> Maybe a 
­­-- Left          :: a ­> Either a b

In a similar way, kinds classify types. For monomorphic types (that is, not polymorphic), the kind signature is just the placeholder *:

­­-- TYPE       KIND 
­­-- [Char]     ::  * 
­­-- Maybe Int  ::  * 

Parametric types express higher-order kinds, for example:

   Maybe  ::  * -> *
   --         a -> Maybe a

where (* -> *) is a placeholder for a -> Maybe a. Let's compare this with the kind signature of Either:

­­-- Either   * ->  * -> * 
­­--              a -> b -> Either a b

The Haskell...