Re: [GHC] #876: stack overflow on 'length . filter odd $ [0 .. 999999999]'

#876: stack overflow on 'length . filter odd $ [0 .. 999999999]'
------------------------------+---------------------------------------------
  Reporter:  ariep@xxxxxxxxx  |          Owner:       
      Type:  bug              |         Status:  new  
  Priority:  normal           |      Milestone:  6.6  
 Component:  Compiler         |        Version:  6.5  
  Severity:  normal           |     Resolution:       
  Keywords:                   |             Os:  Linux
Difficulty:  Unknown          |   Architecture:  x86  
------------------------------+---------------------------------------------
Comment (by igloo):

 I've confirmed that the patch Simon M indicates is indeed the problem, and
 I've copied it below for reference.

 It makes sense to me that the above behaviour is seen: length is now a
 good
 consumer, but it generates 1+(1+(1+(1+... as it is consuming, and this
 causes a stack overflow. I don't think we can fix this while staying with
 fold/build fusion, so it looks to me like the patch should be reverted and
 the whole problem looked at post-6.6.

 Do you agree, Simon PJ?


 Thanks
 Ian

 {{{
 [Make length a good consumer
 simonpj@microsoft**20060508142726

  Make length into a good consumer.  Fixes Trac bug #707.

  (Before length simply didn't use foldr.)

 ] {
 hunk ./GHC/List.lhs 114
 -length                  :: [a] -> Int
 -length l                =  len l 0#
 -  where
 -    len :: [a] -> Int# -> Int
 -    len []     a# = I# a#
 -    len (_:xs) a# = len xs (a# +# 1#)
 +length :: [a] -> Int
 +length l = lenAcc 0# l
 +
 +{-# RULES
 +"length" [~1] forall xs. length xs = foldr incL (I# 0#) xs
 +"lenAcc" [1]  forall n#. foldr incL (I# n#) = lenAcc n#
 + #-}
 +
 +incL :: a -> Int -> Int        -- Internal
 +{-# NOINLINE [0] incL #-}
 +incL x n = n `plusInt` oneInt
 +
 +lenAcc :: Int# -> [a] -> Int   -- Internal
 +lenAcc a# []     = I# a#
 +lenAcc a# (_:xs) = lenAcc (a# +# 1#) xs
 }
 }}}

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/876>
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