[GHC] #7309: The Ix instance for (,) leaks space in range

[GHC]

#7309: The Ix instance for (,) leaks space in range
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 Reporter:  nomeata                  |          Owner:                   
     Type:  bug                      |         Status:  new              
 Priority:  normal                   |      Component:  libraries (other)
  Version:  7.6.1                    |       Keywords:                   
       Os:  Unknown/Multiple         |   Architecture:  Unknown/Multiple 
  Failure:  Runtime performance bug  |       Testcase:                   
Blockedby:                           |       Blocking:                   
  Related:                           |  
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 (at least if you take leak to mean unexpected space behaviour).

 This was brought to my attention via
 http://stackoverflow.com/questions/12780497/puzzling-memory-behavior-in-
 haskell where someone created a 6×10^6-array and was surprised that
 showing the array caused an additional (3+2)*8*10^6 bytes to be used. The
 reason (as far as I could tell) was this code:

 {{{
 instance (Ix a, Ix b) => Ix (a, b) where
     range ((l1,l2),(u1,u2)) = [ (i1,i2) | i1 <- range (l1,u1), i2 <- range
 (l2,u2) ]
 }}}
 in Arr.GHC. The result of {{{range (l2,u2)}}} is shared between every step
 in the first component of the index.

 I guess it is reasonable to expect that the result of range is never worth
 sharing (is it?). I am not entirely sure what the best, cleanest,
 alternative implementation is, though. (At least not without a working
 ```dontshare``` annotation ;-))

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7309>
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