Deprecate an instance

[Jon Fairbairn]

Would it be possible to extend the DEPRECATED pragma to
allow one to deprecate an instance of a class?

I was thinking about the recent discussion of APIs on
haskell-cafe, where Jérémy Bobbio complained about using
Booleans as arguments to libaray functions, preferring
instead sensibly named data constructors. It occurred to me
that by defining a class one could allow both the old
interface and a better one by using a class:

   module Main where

   -- suppose f used to be Bool -> Int -> Int, replace it
   -- with this:

   f :: Toggle t => t -> Int -> Int

   f t x = case enabled t
	   of Invert -> -x
	      DontInvert -> x

   data DoInvert = Invert | DontInvert

   class Toggle t where
       enabled :: t -> DoInvert

   instance Toggle DoInvert where
       enabled = id
   instance Toggle Bool where
       enabled True = Invert
       enabled False = DontInvert


but for this to be any real use, we'd have to be able to
deprecate the /instance/ Toggle Bool.

-- 
Jón Fairbairn                              Jon.Fairbairn at cl.cam.ac.uk