[GHC] #13040: realToFrac into Complex Double has no specialization

28 Dec 2016

      #13040: realToFrac into Complex Double has no specialization
-------------------------------------+-------------------------------------
           Reporter:  akio           |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:                 |           Version:  8.0.1
  libraries/base                     |
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Example:

 {{{#!hs
 module Foo where
 import Data.Complex

 f :: Double -> Complex Double
 f x = realToFrac x
 }}}

 `ghc -O -ddump-simpl` gives:

 {{{
 f =
   \ (w_s17v :: Double) ->
     case w_s17v of _ [Occ=Dead] { GHC.Types.D# ww1_s17y ->
     case GHC.Float.$w$ctoRational ww1_s17y
     of _ [Occ=Dead] { (# ww3_a152, ww4_a153 #) ->
     case GHC.Float.rationalToDouble ww3_a152 ww4_a153
     of dt_a15c { GHC.Types.D# ipv_a15g ->
     Data.Complex.:+ @ Double dt_a15c Data.Complex.$fFloatingComplex1
     }
     }
     }
 }}}

 This means the conversion goes through Rational. However, in this case,
 the conversion could be done much more efficiently with `(:+0)`. It would
 be nice if this happened automatically.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13040
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