[Haskell-cafe] Expanding do notation
haskell at list.mightyreason.com
Sat Jan 7 12:28:37 EST 2006
Josef Svenningsson wrote:
> On 1/7/06, *Chris Kuklewicz* <haskell at list.mightyreason.com
> <mailto:haskell at 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.
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.
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