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