Theory

Folds reduce a list to a single value by combining elements with a binary operator.

name	type	accumulation order
`foldr`	$(a \to b \to b) \to b \to [a] \to b$	right‑associative
`foldl`	$(b \to a \to b) \to b \to [a] \to b$	left‑associative / tail‑recursive

foldr op z []     = z
foldr op z (x:xs) = x `op` foldr op z xs
 
foldl op z []     = z
foldl op z (x:xs) = foldl op (z `op` x) xs

foldr works on infinite lists (lazy, needs only what op demands).
foldl' (strict version) is memory‑friendly for large finite lists. $foldr (+) 0 [1, 2, 3] ⟹ 1 + (2 + (3 + 0))$ $foldl (+) 0 [1, 2, 3] ⟹ ((0 + 1) + 2) + 3$

Implementation

sumR  = foldr (+) 0
sumL  = foldl' (+) 0
 
reverse = foldl (flip (:)) []