Fwd: [Haskell-cafe] curious about sum

Once more I forgot to send my messages to the haskell cafe list. All the rest of the list which I´m suscribed to, send the mail replies to the list automatically, but this doesn´t. Please, can this be changed?. ---------- Forwarded message ---------- From: Alberto G. Corona Date: 2009/6/13 Subject: Re: [Haskell-cafe] curious about sum To: Deniz Dogan first, I was completely wrong. It is foldl what is neccesary. sum is defined in terms of foldl: sum= foldl (+) 0 but Prelude> foldl (+) 0 [1..1000000] *** Exception: stack overflow that is not because + is non strict, but because foldl is: foldl f z0 xs0 = lgo z0 xs0 where lgo z [] = z lgo z (x:xs) = lgo (f z x) xs this version of foldl IS strict: foldlStrict f z0 xs0 = lgo z0 xs0 where lgo z [] = z lgo z (x:xs) =let t= f z x in t `seq` lgo t xs main= print $ foldlStrict (+) 0 [1..1000000] 500000500000 so the garbage collector do the job in freeing the consumed part of the list. 2009/6/13 Alberto G. Corona

Prelude> let strictplus x y= let z=x +y in z `seq` z; sum1= foldr

*** Exception: stack overflow I suppose that strictplus is strict, so the garbage collector would free

strictplus 0 in sum1[0..1000000] the consumed part of the list.

Then, why the stack overflow? 2009/6/13 Deniz Dogan

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2009/6/13 Jochem Berndsen :

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Keith Sheppard wrote:

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Is there any reason that sum isn't strict? I can't think of any case where that is a good thing.

Prelude> sum [0 .. 1000000] *** Exception: stack overflow

It is useful if the (+) is nonstrict; although I cannot think of any useful mathematical structure where (+) would be nonstrict.

I remember needing a non-strict sum at least once, but I do not remember the exact application. But imagine having a (very) long list of numbers and you want to do A if the sum exceeds a small number, otherwise B.

if sum [0..100000] > 10 then A else B

However, this idea didn't work, because of strictness.

-- Deniz Dogan _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe