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 <daniel.is.fischer@web.de> 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.