[GHC] #2076: rational infinities don't compare correctly to each other

[GHC]

#2076: rational infinities don't compare correctly to each other
---------------------------------+------------------------------------------
    Reporter:  uhollerbach       |       Owner:  jeffrey         
        Type:  bug               |      Status:  new             
    Priority:  normal            |   Milestone:  _|_             
   Component:  libraries/base    |     Version:  6.8.2           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:                    |  
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Comment(by jeffrey):

 In libraries/base/GHC/Real.lhs, I added

 {{{
 instance  (Integral a)  => Ord (Ratio a)  where
     {-# SPECIALIZE instance Ord Rational #-}

 +   (x:%0) <= (x':%0 )  =  x      <  0 || 0 < x'
 +   (x:%0) <= (_ :%_ )  =  x      <  0
 +   (_:%_) <= (x':%0 )  =  0      <  x'
     (x:%y) <= (x':%y')  =  x * y' <= x' * y

 +   (x:%0) <  (x':%0 )  =  x      <  0 && 0 < x'
 +   (x:%0) <  (_ :%_ )  =  x      <  0
 +   (_:%_) <  (x':%0 )  =  0      <  x'
     (x:%y) <  (x':%y')  =  x * y' <  x' * y
 }}}

 which passes my testsuite for this bug. However, I also tested the
 equivalence of same-sign infinities (which I assumed to be a desired
 property when I wrote the code) and found that it wasn't working; for
 example

 {{{
 Prelude GHC.Real> (2:%0) == (1:%0)
 False
 }}}

 When I also found that, for example,

 {{{
 Prelude GHC.Real> (2:%4) == (1:%2)
 False
 }}}

 I figured that (:%) wasn't intended to equate different forms of actually-
 equal ratios. So I figured my proposed bug fix would just introduce
 another bug, since it, for example, yields

 {{{
 Prelude GHC.Real> (2:%0) < (1:%0) -- should be False
 False
 Prelude GHC.Real> (2:%0) <= (1:%0)
 True
 }}}

 which contradicts the non-equality observed above. But then I tried

 {{{
 Prelude GHC.Real> compare (2:%4) (1:%2)
 LT
 Prelude GHC.Real> (2:%4) < (1:%2)
 False
 }}}

 which doesn't make sense(?), and figured I'm probably missing the bigger
 picture here. Thoughts?

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2076#comment:6>
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