[GHC] #12708: RFC: Representation polymorphic Num

[GHC]

#12708: RFC: Representation polymorphic Num
-------------------------------------+-------------------------------------
        Reporter:  Iceland_jack      |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Core Libraries    |              Version:  8.0.1
      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 Iceland_jack):

 Using the generalized function composition from the paper

 {{{#!hs
 (·) :: forall (r :: RuntimeRep) (a :: Type) (b :: Type) (c :: TYPE r).
        (b -> c)
     -> (a -> b)
     -> (a -> c)
 (f · g) x = f (g x)
 }}}

 we can write

 {{{#!hs
   fromInteger :: Integer -> Int#
   fromInteger (fromInteger -> I# i) = i

   fromInteger :: Integer -> Int#
   fromInteger (fromInteger -> D# d) = d)
 }}}

 pointfree

 {{{#!hs
   fromInteger :: Integer -> Int#
   fromInteger = getInt# · fromInteger where
     getInt# :: Int -> Int#
     getInt# (I# i) = i

   fromInteger :: Integer -> Double#
   fromInteger = getDouble# · fromInteger where
     getDouble# :: Double -> Double#
     getDouble# (D# d) = d

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12708#comment:19>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler