Space leak

[Christian Maeder]

Tomasz Zielonka wrote:
> On Tue, Dec 27, 2005 at 08:12:20PM +0100, Christian Maeder wrote:
>> Already the following bit exhibits unexpected memory consumption:
>>
>> main = mapM_ print $ take (n * 5) $ drop (n * 3) [1..]
>> n = 100000
> 
> I think it's the (succ (succ (succ ...))) thunk. When you change it to
> [1..99999999] there is no space leak - here every produced element must
> be examined before producing the next list node or [].

ok, I could also fix the space leak by defining my own iterate function:

myiterate :: (a -> a) -> a -> [a]
myiterate f a = a : (myiter f $! f a)

main =
    mapM_ print $ take (n * 5) $ drop (n * 3) $ myiterate succ 1

is also ok.

The next problem occurs when I try to use splitAt as in the original DNA 
example:

main = do
   let (s1, s2) = splitAt (n * 3) $ myiterate succ 1
   mapM_ print s1
   mapM_ print $ take (n * 5) s2

The above code leaks space! But

main = do
   let s1 = take (n * 3) $ myiterate succ 1
       s2 = drop (n * 3) $ myiterate succ 1
   mapM_ print s1
   mapM_ print $ take (n * 5) s2

does not leak, as long as it is compiled without optimization.
(Eliminating the CSE "myiterate succ 1" makes it leak.)

Could optimization be improved?

Cheers Christian