[GHC] #9585: Note about semantics of (!!) is not specific enough, if it's correct

[GHC]

#9585: Note about semantics of (!!) is not specific enough, if it's correct
-------------------------------------+-------------------------------------
       Reporter:  dfeuer             |                   Owner:
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:
      Component:  libraries/base     |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Unknown            |         Type of failure:
     Blocked By:                     |  Documentation bug
Related Tickets:                     |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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 The definition of `(!!)` in `GHC.List`, copied below, has a vague note
 about semantics for the Prelude and HBC versions being different. I can't
 see the difference myself. If there really is a difference, the details
 should be in the comment.

 {{{#!hs
 -- | List index (subscript) operator, starting from 0.
 -- It is an instance of the more general 'Data.List.genericIndex',
 -- which takes an index of any integral type.
 (!!)                    :: [a] -> Int -> a
 #ifdef USE_REPORT_PRELUDE
 xs     !! n | n < 0 =  error "Prelude.!!: negative index"
 []     !! _         =  error "Prelude.!!: index too large"
 (x:_)  !! 0         =  x
 (_:xs) !! n         =  xs !! (n-1)
 #else
 -- HBC version (stolen), then unboxified
 -- The semantics is not quite the same for error conditions
 -- in the more efficient version.
 --
 xs !! (I# n0) | isTrue# (n0 <# 0#) =  error "Prelude.(!!): negative
 index\n"
               | otherwise          =  sub xs n0
                          where
                             sub :: [a] -> Int# -> a
                             sub []     _ = error "Prelude.(!!): index too
 large\n"
                             sub (y:ys) n = if isTrue# (n ==# 0#)
                                            then y
                                            else sub ys (n -# 1#)
 #endif
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9585>
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