Re: [Haskell-cafe] foldl vs foldl'
Bas van Dijk wrote: On Wed, Nov 5, 2008 at 12:43 AM, Bas van Dijk [EMAIL PROTECTED] wrote: 2008/11/5 Daryoush Mehrtash [EMAIL PROTECTED]: Are there cases (function or list) where the result of foldl (or foldr)would be different that foldl' (or foldr')? Maybe this wiki article I wrote some time ago will answer your question: http://haskell.org/haskellwiki/Foldr_Foldl_Foldl' Oops that link should be: http://haskell.org/haskellwiki/Foldr_Foldl_Foldl%27 I have an idea for the foldl diagram. If you rotate the RHS 90deg clockwise, it slopes the same way as the original list, but the 'f's are still rotated sideways to signify which their inputs are. That makes the relationship between imperative accumulator loops and foldl a bit clearer to me, as well as the relationship between the list and the recursion structure. -- src/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldl vs foldl'
Excerpts from daniel.is.fischer's message of Wed Nov 05 00:37:47 +0100 2008: Am Mittwoch, 5. November 2008 00:08 schrieb Daryoush Mehrtash: Are there cases (function or list) where the result of foldl (or foldr)would be different that foldl' (or foldr')? thanks, daryoush Simple example: import Data.List weird :: Int - Int - Int weird _ 0 = 0 weird x y = x*y list :: [Int] list = [1, 2, 3, 4, undefined, 6, 7, 8, 9, 0] okey = foldl weird 1 list boom = foldl' weird 1 list *Main okey 0 *Main boom *** Exception: Prelude.undefined since foldl' evaluates strictly (to WHNF), it can die on encountering an undefined value in the list where foldl doesn't. Your example is a nice example of foldl over foldl', it would be nice to have it in the wiki page about the different folds[1]. Best regards, [1]: http://haskell.org/haskellwiki/Foldr_Foldl_Foldl%27 -- Nicolas Pouillard aka Ertai ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldl vs foldl'
Lets assume we don't have undefined in the list, are there functions (or properties in the function) that would cause foldl to have different results than foldl'? daryoush On Tue, Nov 4, 2008 at 3:37 PM, Daniel Fischer [EMAIL PROTECTED]wrote: Am Mittwoch, 5. November 2008 00:08 schrieb Daryoush Mehrtash: Are there cases (function or list) where the result of foldl (or foldr)would be different that foldl' (or foldr')? thanks, daryoush Simple example: import Data.List weird :: Int - Int - Int weird _ 0 = 0 weird x y = x*y list :: [Int] list = [1, 2, 3, 4, undefined, 6, 7, 8, 9, 0] okey = foldl weird 1 list boom = foldl' weird 1 list *Main okey 0 *Main boom *** Exception: Prelude.undefined since foldl' evaluates strictly (to WHNF), it can die on encountering an undefined value in the list where foldl doesn't. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldl vs foldl'
On Wed, 2008-11-05 at 10:01 -0800, Daryoush Mehrtash wrote: Lets assume we don't have undefined in the list, are there functions (or properties in the function) that would cause foldl to have different results than foldl'? If the function is partial on some elements of the list. (3 /), for example, if the list contains 0. If f is total over the elements of the list (whether the elements of the list are partial or total) and f z /= _|_, then foldl' f z = foldl f z. jcc ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldl vs foldl'
On Wed, 2008-11-05 at 10:01 -0800, Daryoush Mehrtash wrote: Lets assume we don't have undefined in the list, are there functions (or properties in the function) that would cause foldl to have different results than foldl'? The only difference in the definition of foldl and foldl' is a seq so it can only differ due to bottoms as far as semantics is concerned. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldl vs foldl'
On Tue, 2008-11-04 at 15:08 -0800, Daryoush Mehrtash wrote: Are there cases (function or list) where the result of foldl (or foldr)would be different that foldl' (or foldr')? There is no foldr'. And yes, foldl and foldl' are different functions and thus return different results on some inputs; however, you almost always want foldl' (v. foldl). See http://www.haskell.org/haskellwiki/Stack_overflow ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldl vs foldl'
Am Mittwoch, 5. November 2008 00:08 schrieb Daryoush Mehrtash: Are there cases (function or list) where the result of foldl (or foldr)would be different that foldl' (or foldr')? thanks, daryoush Simple example: import Data.List weird :: Int - Int - Int weird _ 0 = 0 weird x y = x*y list :: [Int] list = [1, 2, 3, 4, undefined, 6, 7, 8, 9, 0] okey = foldl weird 1 list boom = foldl' weird 1 list *Main okey 0 *Main boom *** Exception: Prelude.undefined since foldl' evaluates strictly (to WHNF), it can die on encountering an undefined value in the list where foldl doesn't. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldl vs foldl'
2008/11/5 Daryoush Mehrtash [EMAIL PROTECTED]: Are there cases (function or list) where the result of foldl (or foldr)would be different that foldl' (or foldr')? Maybe this wiki article I wrote some time ago will answer your question: http://haskell.org/haskellwiki/Foldr_Foldl_Foldl' regards, Bas ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] foldl vs foldl'
On Wed, Nov 5, 2008 at 12:43 AM, Bas van Dijk [EMAIL PROTECTED] wrote: 2008/11/5 Daryoush Mehrtash [EMAIL PROTECTED]: Are there cases (function or list) where the result of foldl (or foldr)would be different that foldl' (or foldr')? Maybe this wiki article I wrote some time ago will answer your question: http://haskell.org/haskellwiki/Foldr_Foldl_Foldl' Oops that link should be: http://haskell.org/haskellwiki/Foldr_Foldl_Foldl%27 ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
