[GHC] #9742: The default definitions of foldl1 and foldr1 are too strict

29 Oct 2014

      #9742: The default definitions of foldl1 and foldr1 are too strict
-------------------------------------+-------------------------------------
       Reporter:  dfeuer             |                   Owner:  ekmett
           Type:  bug                |                  Status:  new
       Priority:  normal             |               Milestone:  7.10.1
      Component:  Core Libraries     |                 Version:  7.9
       Keywords:                     |        Operating System:
   Architecture:  Unknown/Multiple   |  Unknown/Multiple
     Difficulty:  Easy (less than 1  |         Type of failure:  Runtime
  hour)                              |  crash
     Blocked By:                     |               Test Case:
Related Tickets:                     |                Blocking:
                                     |  Differential Revisions:
-------------------------------------+-------------------------------------
 We currently have

 {{{#!hs
     foldr1 :: (a -> a -> a) -> t a -> a
     foldr1 f xs = fromMaybe (error "foldr1: empty structure")
                     (foldr mf Nothing xs)
       where
         mf x Nothing = Just x
         mf x (Just y) = Just (f x y)
 }}}

 This has to traverse the entire list before it can do anything. What we
 want is

 {{{#!hs
 mf x r = Just $ case r of
                   Nothing -> x
                   Just y -> f x y
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9742
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