Re: [Haskell-cafe] How do I get this done in constant mem?

On Sat, Oct 10, 2009 at 09:33:52AM -0700, Thomas Hartman wrote:

To: Luke Palmer Cc: mf-hcafe-15c311f0c@etc-network.de, haskell-cafe@haskell.org From: Thomas Hartman Date: Sat, 10 Oct 2009 09:33:52 -0700 Subject: Re: [Haskell-cafe] How do I get this done in constant mem?

...

Yes, you should not do this in IO. That requires the entire computation to finish before the result can be used.

Not really the entire computation though... whnf, no?

In that example, yes. But readFile takes the entire file into a strict String before it gives you the first Char, right? (Sorry again for my misleading code "simplification".)

main = do let thunks :: IO [Int] thunks = (sequence . replicate (10^6) $ (randomRIO (0,10^9))) putStrLn . show . head =<< thunks -- prints putStrLn . show . last =<< thunks -- overflows

Meaning that the entire list needs to be kept? Is there a reason (other than "it's easier to implement and it's legal" :-) why the elements that have been traversed by "last" can't be garbage collected? -m