[Haskell-cafe] External Sort: Sort a 10-million integer file with just 256M of ram.

Albert Y. C. Lai trebla at vex.net
Sat Oct 25 14:02:14 EDT 2008

Bulat Ziganshin wrote:
> Hello Thomas,
> Thursday, October 23, 2008, 8:41:04 PM, you wrote:
>> The problem is not fitting a 10^8 element list in memory, the
>> following works fine
>>  (when compiled, though not in ghci):
>> t = putStrLn . show . last $ [1..10^8::Int]
> this runs in 1k space, thanks to lazy evaluation. 10^8-length list
> needs ~3gb of memory, it was discussed just a few days ago

To elaborate, t does this: compute the next item in the list, throw the 
previous item away, until there is no next item, now we have something 
to print. We never keep the whole list.

But this may keep the whole list:

u = (putStrLn . show . last $ list) >> (putStrLn . show . head $ list)
   where list = [1..10^8::Int]

Have fun!

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