[GHC] #8539: Data.Complex shouldn't use default implementation of (**)

[GHC]

#8539: Data.Complex shouldn't use default implementation of (**)
-------------------------------------+-------------------------------------
        Reporter:  jjaredsimpson     |            Owner:
            Type:  bug               |           Status:  patch
        Priority:  low               |        Milestone:
       Component:  Prelude           |          Version:  7.6.3
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  Incorrect result  |       Difficulty:  Easy (less than 1
  at runtime                         |  hour)
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by yalas):

 What about just adding check x>=0?
 {{{
 (x:+0) ** (y:+0)
   | x >= 0     =  (x ** y) :+ 0
 x ** y         =  exp (log x * y)
 }}}
 It will be more accurate when raising real numbers to real power:
 {{{
 Prelude Data.Complex> exp (log (2) * 4) :: Complex Double
 15.999999999999998 :+ 0.0
 Prelude Data.Complex> 2**4 :: Double
 16.0
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8539#comment:6>
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