Re: [Haskell-cafe] Expanding do notation

Josef Svenningsson wrote:

On 1/7/06, *Chris Kuklewicz* mailto:haskell@list.mightyreason.com> wrote:

When you put "print (head p)" at then end, it keeps a reference to the whole list "p" which is your space leak. If you want to store the head of p, this *should* work:

> main = do n <- getArgs >>= return . read . head > let p = permutations [1..n] > headOfP <- return (head p) > mapM_ (putStrLn . concatMap show) $ take 30 p > putStr $ "Pfannkuchen(" ++ show n ++ ") = " > putStrLn . show $ foldl' (flip (max . steps 0)) 0 p > print headOfP

The "headOfP" is computed as an IO action at that point in the program, so "headOfP" does not hold a reference to "p" as a whole.

Without having tried this I'd say that you should use Control.Excection.evaluate instead of 'return'. Or you could use 'seq'. I suspect that otherwise the 'head p' will not be evaluated until the last line and you will have the same problem as before.

Cheers,

/Josef

Thanks Josef. headOfP <- return $! (head p) headOfP <- (head p) `seq` return (head p) headOfP <- return (Control.Excection.evaluate (head p)) Are strict enough to do the trick.