fold_examples.hs

reverse' :: [a] -> [a]
reverse' = foldl (\acc x -> x : acc) []

reverse'' :: [a] -> [a]
reverse'' = foldl (flip (:)) []

product' :: (Num a) => [a] -> a
product' = foldl (*) 1

filter' :: (a -> Bool) -> [a] -> [a]
filter' p = foldr (\x acc -> if p x then x : acc else acc) []

last' :: [a] -> a
last' = foldl1 (\_ x -> x)

main = do
  print $ reverse' [1,2,3,4,5]
  print $ reverse'' [1,2,3,4,5]
  print $ product' [1,2,3,4,5]
  print $ filter' (< 3) [1,2,3,4,5]
  print $ last' [1,2,3,4,5]