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
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.