It was discussed a bit here: http://ghc.haskell.org/trac/ghc/ticket/8090 Rank N Kinds: Main Idea is: If we assume an infinite hierarchy of classifications, we have True :: Bool :: * :: ** :: *** :: **** :: ... Bool = False, True, ... * = Bool, Sting, Maybe Int, ... ** = *, *->Bool, *->(*->*), ... *** = **, **->*, (**->**)->*, ... ... RankNKinds is also a part of lambda-cube. PlyKinds is just type of ** (Rank2Kinds) class Foo (a :: k) where <<==>> class Foo (a :: **) where *** is significant to work with ** data and classes; more general: Rank(N)Kinds is significant to work with Rank(N-1)Kinds First useful use is in Typeable. In GHC 7.8 class Typeable (a::k) where ... <<==>> class Typeable (a ::**) where ... But we can't write data Foo (a::k)->(a::k)->* ... deriving Typeable If we redeclare class Typeable (a ::***) where ... or even class Typeable (a ::******) where ... it becomes far enough for many years I'm asking to find other useful examples -- View this message in context: http://haskell.1045720.n5.nabble.com/Rank-N-Kinds-tp5733482.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.