Haskell - implement map and filter functions by recursion and foldr versions.

map_filter.hs

-- | recursion version

map' :: (a -> b) -> [a] -> [b]
map' _ [] = []
map' f (x:xs) = f x : map' f xs

filter' :: (a -> Bool) -> [a] -> [a]
filter' _ [] = []
filter' f (x:xs)
  | f x       = x : (filter' f xs)
  | otherwise = filter' f xs


-- | foldr version

map'' :: (a -> b) -> [a] -> [b]
map'' f = foldr (\x acc -> (f x) : acc) []

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