すごいH本の再帰関数あたり #CodePiece

CodePiece.hs

zip' :: [a] ->[b] -> [(a,b)]
zip' _ [] = []
zip' [] _ = []
zip' (x:xs) (y:ys) = (x,y) : zip' xs ys


elem' :: (Eq a) => a -> [a] -> Bool
elem' a [] = False
elem' a (x:xs)
  | a == x    = True
  | otherwise = a `elem'` xs

quicksort :: (Ord a) => [a] -> [a]
quicksort [] = []
quicksort (x:xs) =
  let smallOrEqual = [a | a <- xs, a <= x]
      larger = [a | a <- xs, a > x]
  in quicksort smallOrEqual ++ [x] ++ quicksort larger