fold、$演算子、関数合成など。書く(写経)だけでも、より畳み込みの動きとか分かるなぁ。 #CodePiece

CodePiece.hs

sum' :: (Num a) => [a] -> a
sum' xs = foldl (\acc x -> acc + x) 0 xs

sum'' :: (Num a) => [a] -> a
sum'' = foldl (+) 0

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


elem' :: (Eq a) => a -> [a] -> Bool
elem' y ys = foldr (\x acc -> if x == y then True else acc) False ys

maximum' :: (Ord a) => [a] -> a
maximum' = foldl1 max

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)

and' :: [Bool] -> Bool
and' xs = foldr (&&) True xs

sqrtSum :: Int
sqrtSum = length (takeWhile (<1000) (scanl1 (+) (map sqrt [1..]))) + 1

oddSquareSum :: Integer
oddSquareSum = sum . takeWhile (<10000) . filter odd $ map (^2) [1..]