[GHC] #9398: Data.List.cycle is not a good producer

[GHC]

#9398: Data.List.cycle is not a good producer
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              Reporter:  dfeuer      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.8.4
             Component:              |          Version:  7.8.3
  libraries/base                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Easy (less than 1
  Unknown/Multiple                   |  hour)
       Type of failure:  Runtime     |       Blocked By:
  performance bug                    |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by dfeuer):

 Replying to [comment:16 nomeata]:
 > >  Yes, it actually cycles the argument and then maps over the
 result—horrible in every way.
 >
 > But this is well-known: You usually cannot “modify” cyclic data
 structures without breaking them.
 >
 > Of course its cool that with the above definition of `cycle` + fusion,
 we can suddenly `map` over a cyclic structure without breaking it, but
 maybe that’s a tad too much magic? Especially as people probably don’t
 have a good feeling for when this happens and when not? OTOH, I don’t
 think it hurts either, and nice surprises are – well – nice.

 Well, I agree that magic can be a problem, but if it falls out of
 something that tends to be good, that's not a major issue.


 > Did you investigate why `foldr (+) 0 $ take 30000000 $ map (* 13) $
 cycle [1,8,4,0,(5::Int)]` allocates more?

 I'm not entirely sure, but it looks to me like it's all about unboxing.
 Somehow, the Prelude version is able to unbox all the `Int`s, whereas this
 one is not. If I replace the `(+)` with

 {{{#!hs
 {-# NOINLINE g #-}
 g a b = a
 }}}

 then both versions run in constant space.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9398#comment:17>
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