[GHC] #4157: A bug of dropWhileR in Data.Sequence

[GHC]

#4157: A bug of dropWhileR in Data.Sequence
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    Reporter:  kazu-yamamoto      |        Owner:              
        Type:  bug                |       Status:  new         
    Priority:  normal             |    Milestone:              
   Component:  libraries (other)  |      Version:  6.12.3      
    Keywords:                     |   Difficulty:              
          Os:  Unknown/Multiple   |     Testcase:              
Architecture:  Unknown/Multiple   |      Failure:  None/Unknown
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Description changed by igloo:

Old description:

> The document of dropWhileR says: dropWhileR  p xs is equivalent to
> reverse  (dropWhileL  p (reverse  xs)).
>
> However,
>
> reverse $ dropWhileL (==4) $ reverse $ fromList [1,2,3,4]
> ==> fromList [1,2,3]
>
> dropWhileR (==4) $ fromList [1,2,3,4]
> ==> fromList [1,2]
>
> I guess the cause of bug is in breakl.
>
> Original:
> breakl p xs = foldr (\ i _ -> S.splitAt i xs) (xs, empty) (findIndicesL p
> xs)
>
> Fixed:
> breakl p xs = foldr (\ i _ -> S.splitAt (i+1) xs) (xs, empty)
> (findIndicesL p xs)

New description:

 The document of dropWhileR says: dropWhileR  p xs is equivalent to reverse
 (dropWhileL  p (reverse  xs)).

 However,
 {{{
 reverse $ dropWhileL (==4) $ reverse $ fromList [1,2,3,4]
 ==> fromList [1,2,3]

 dropWhileR (==4) $ fromList [1,2,3,4]
 ==> fromList [1,2]
 }}}
 I guess the cause of bug is in breakl.

 Original:
 {{{
 breakl p xs = foldr (\ i _ -> S.splitAt i xs) (xs, empty) (findIndicesL p
 xs)
 }}}
 Fixed:
 {{{
 breakl p xs = foldr (\ i _ -> S.splitAt (i+1) xs) (xs, empty)
 (findIndicesL p xs)
 }}}

--

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/4157#comment:1>
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