[GHC] #10535: Float out causes major space leak

[GHC]

#10535: Float out causes major space leak
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              Reporter:  AlexET      |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.1
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 This issue originally was on haskell-cafe
 https://mail.haskell.org/pipermail/haskell-cafe/2015-June/120113.html

 The following code has a space leak

 {{{
 #!hs
 numbers = [1..200]

 replicateM' :: Monad m => Int -> m a -> m [a]
 replicateM' 0 xs = return []
 replicateM' n xs = do
   a <- xs
   b <- replicateM' (n-1) xs
   return (a:b)

 test :: [[Int]]
 test = replicateM' 4 numbers

 main = print test
 }}}

 The recursive call in replicateM' gets floated out. This then causes a
 very large (200^3 elements) list to be kept in memory instead of being
 lazily produced and consumed each time.

 A similar thing happens to the replicateM in Control.Monad but it is
 harder to spot as a lot of specialisation and fusion goes on before the
 float-out happens

 This is similar to #7367 but in this case we have a huge space-leak
 instead of just adding allocations.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10535>
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