[GHC] #13080: Memory leak caused by nested monadic loops

[GHC]

#13080: Memory leak caused by nested monadic loops
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        Reporter:  Feuerbach         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by simonpj):

 Suppose it had been
 {{{
 worker :: (Monad m) => m ()
 worker = forever $ poll 1

 poll :: (Monad m) => Int -> m a
 poll n
  | n>10000    = return ()
  | otherwise = do print (expensive n)
                   poll (n+1)
 }}}
 where `expensive` is expensive to compute.  After printing out the results
 of 10000 calls to `expensive`, the `forever` will do it all again.

 Question: would you expect all the calls to `expensive` to be recomputed?
 Presumably not.  `poll` builds a big action
 {{{
 print (expensive 0) >> print (expensive 1) >> print (expensive 2) >> ...
 }}}
 and `forever` just repeatedly executes that action.  But to remember all
 those results clearly takes O(n) space.

 Now in this case there is no real work to be shared, but that's clearly
 harder for GHC to spot.  Especially when (as in this case) the monad is
 unspecified, so perhaps the call `return ()` does a tremendous amount of
 work.

 I'm not saying things can't be improved here, but you are in delicate
 territory.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13080#comment:14>
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