Why foldl' giving an error while foldl working fine in the same definition

January 13, 2025

Here i am trying to reimplement a safe maximum using folds

import Data.ByteString (foldl')

maximum' :: Ord a => [a] -> Maybe a
maximum'  = foldl  (\ acc x -> max acc (Just x)) Nothing

maximum'' :: Ord a => [a] -> Maybe a
maximum'' = foldl' (\ acc x -> max acc (Just x)) Nothing

First function that uses foldl working correctly but second one gives this error:

• Couldn’t match expected type ‘a’
with actual type ‘GHC.Word.Word8’
‘a’ is a rigid type variable bound by
the type signature for:
maximum” :: forall a. Ord a => [a] -> Maybe a
at /home/mali/Projects/Deneme/HigherOrder.hs:60:1-36
• In the first argument of ‘Just’, namely ‘x’
In the second argument of ‘max’, namely ‘(Just x)’
In the expression: max acc (Just x)

Shouldn’t foldl’ be just a more efficient version of foldl?

>Solution :

You want the foldl' from Prelude (which works on lists), not Data.ByteString (which works on bytestrings). You don’t have to import anything since Prelude is imported automatically.