Hello, Is there any reason other than history that foldl and foldl' are not defined in terms of foldr? If we define them in terms of foldr like: foldl f z xs = foldr (\x y -> \z' -> let z'' = z' `f` x in y z'') id xs z {-# INLINE foldl #-} foldl' f z xs = foldr (\x y -> \z' -> let !z'' = z' `f` x in y z'') id xs z {-# INLINE foldl' #-} we can benefit from list fusion. For example if we define sum as: sum :: Num a => [a] -> a sum = foldl (+) 0 then building the following program with -O2: fuse = sum (replicate 1000000 1 ++ replicate 5000 1 :: [Int]) yields the following totally fused core: fuse :: Int fuse = case $wxs 1000000 0 of ww_ssn { __DEFAULT -> I# ww_ssn } $wxs :: Int# -> Int# -> Int# $wxs = \ (w_ssg :: Int#) (ww_ssj :: Int#) -> case <=# w_ssg 1 of _ { False -> $wxs (-# w_ssg 1) (+# ww_ssj 1); True -> $wxs1_rsB 5000 (+# ww_ssj 1) } $wxs1_rsB :: Int# -> Int# -> Int# $wxs1_rsB = \ (w_ss5 :: Int#) (ww_ss8 :: Int#) -> case <=# w_ss5 1 of _ { False -> $wxs1_rsB (-# w_ss5 1) (+# ww_ss8 1); True -> +# ww_ss8 1 } Regards, Bas