I am trying to find an explanation of the DataKinds extension that will make sense to me having come from only having read [Learn You a Haskell][1]. Is there a standard source that will make sense to me with what little I&#39;ve learned?

Edit: For example the [documentation][2] says

&gt; With -XDataKinds, GHC automatically promotes every suitable datatype
&gt; to be a kind, and its (value) constructors to be type constructors.
&gt; The following types

and  gives the example

    data Nat = Ze | Su Nat

give rise to the following kinds and type constructors:

    Nat :: BOX
    Ze :: Nat
    Su :: Nat -&gt; Nat

I am not getting the point. Although I don&#39;t understand what `BOX` means, the statements `Ze :: Nat` and `Su :: Nat -&gt; Nat` seem to state what is already normally the case that Ze and Su are normal data constructors exactly as you would expect to see with ghci

    Prelude&gt; :t Su
    Su :: Nat -&gt; Nat


  [1]: http://learnyouahaskell.com/
  [2]: http://www.haskell.org/ghc/docs/7.6.2/html/users_guide/promotion.html

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What is the DataKinds extension of Haskell?