haskell基础复习巩固

Review.hs

-- fold left
foldLeft :: (b -> a -> b) -> b -> [a] -> b
foldLeft _ acc [] = acc
foldLeft f acc (x : xs) = foldLeft f (f acc x) xs

-- fold right
foldRight :: (a -> b -> b) -> b -> [a] -> b
foldRight f acc [] = acc
foldRight f acc (x : xs) = f x (foldRight f acc xs)

length' :: [a] -> Int
length' = foldLeft (\acc _ -> acc +1) 0 

sum' = foldLeft (+) 0

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

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

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


(+=+) :: [a] -> [a] -> [a]

xs +=+ ys = foldRight (:) ys xs

zip' :: [a] -> [b] -> [(a,b)]

zip' (x : xs) (y : ys) = (x , y) : zip' xs ys
zip' _        _ = []