Re: Fwd: [Haskell-cafe] curious about sum
On Sun, Jun 14, 2009 at 21:23, Jochem Berndsenjoc...@functor.nl wrote: Alberto G. Corona wrote: Once more I forgot to send my messages to the haskell cafe list. All the rest of the list which I惴 suscribed to, send the mail replies to the list automatically, but this doesn��. Please, can this be changed?. This comes up every so often, but I would be against this. Your e-mail client should support mailing lists. This list does not fill out the Reply-To header. Many other lists do. I am all for adding it, if there's no specific reason not to. Thomas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Fwd: [Haskell-cafe] curious about sum
Again: what if somebody wants to answer to the original author privately? It's easier to just use Reply for private answers and Reply all for list answers. Thomas ten Cate wrote on 15.06.2009 11:18: On Sun, Jun 14, 2009 at 21:23, Jochem Berndsenjoc...@functor.nl wrote: Alberto G. Corona wrote: Once more I forgot to send my messages to the haskell cafe list. All the rest of the list which I惴 suscribed to, send the mail replies to the list automatically, but this doesn��. Please, can this be changed?. This comes up every so often, but I would be against this. Your e-mail client should support mailing lists. This list does not fill out the Reply-To header. Many other lists do. I am all for adding it, if there's no specific reason not to. Thomas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
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 agocor...@gmail.com Date: 2009/6/13 Subject: Re: [Haskell-cafe] curious about sum To: Deniz Dogan deniz.a.m.do...@gmail.com 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..100] *** 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..100] 5050 so the garbage collector do the job in freeing the consumed part of the list. 2009/6/13 Alberto G. Corona agocor...@gmail.com Prelude let strictplus x y= let z=x +y in z `seq` z; sum1= foldr strictplus 0 in sum1[0..100] *** Exception: stack overflow I suppose that strictplus is strict, so the garbage collector would free the consumed part of the list. Then, why the stack overflow? 2009/6/13 Deniz Dogan deniz.a.m.do...@gmail.com 2009/6/13 Jochem Berndsen joc...@functor.nl: Keith Sheppard wrote: 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 .. 100] *** 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..10] 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 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Fwd: [Haskell-cafe] curious about sum
Alberto G. Corona wrote: 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?. This comes up every so often, but I would be against this. Your e-mail client should support mailing lists. 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..100] 5050 so the garbage collector do the job in freeing the consumed part of the list This is correct; the function you defined is equivalent to foldl' in Data.List, if I'm not mistaken. Regards, -- Jochem Berndsen | joc...@functor.nl GPG: 0xE6FABFAB ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
