[GHC] #15183: Expose the SNat type and the natSing method

[GHC]

#15183: Expose the SNat type and the natSing method
-------------------------------------+-------------------------------------
        Reporter:  dfeuer            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Core Libraries    |              Version:  8.2.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by goldfire):

 This was a design decision. In 2012 or so, Iavor was developing the
 TypeLits interface simultaneously with my development of singletons. We
 wanted to have one `Sing`, shared between the TypeLits stuff (built into
 GHC) and the singletons stuff (external). But the design considerations
 pulled at each other -- especially because `singletons` could make changes
 much more quickly than GHC could. So, instead of having built-in
 singletons and external singletons, it was decided to hide the built-in
 ones.

 Maybe now that we have more experience, this decision could be updated...
 but I still think that introducing any singletons into `base` will start a
 chain reaction of other feature requests.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15183#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler