Specifying Kinds of Types

[Ross Paterson]

On Fri, Feb 08, 2002 at 12:39:30PM +0100, Rijk J.C.van Haaften wrote:
> Ashley Yakeley wrote:
> >I'd like to be able to declare the kinds of new types and synonyms,
> >because sometimes Haskell can't infer them.
> 
> It is possible using a trick due to John Hughes. In
>     Proceedings of the 1999 Haskell Workshop,
> he wrote in his article
>     Restricted Data Types in Haskell
> [...]
> 
>     data Set cxt a = Set [a] | Unused (cxt a -> ()) deriving Show

Another kludge, that works with newtypes too, is

type Hint a b = a
newtype Set cxt a = Set (Hint [a] (cxt a)) deriving Show