Haskell始めました - ria0130’s blog

length' [] = 0
length'  (_:xs) = 1 + length' xs

sum' [] = 0
sum' (x:xs) = x + sum' xs

product' [] = 1
product' (x:xs) = x * product' xs

take' _ [] = []
take' n _ | n < 1 = []
take' n (x:xs) = x : take' (n - 1) xs

drop' _ [] = []
drop' n xs | n < 1 = xs
drop' n (_:xs) = drop' (n - 1) xs

reverse' [] = []
reverse' (x:xs) = reverse' xs ++ [x]


main = do
  -- リストの長さ
  print $ length' [1,2,3,4,5]  -- 5

  -- 総和
  print $ sum' [1,2,3,4,5] -- 15

  -- 階上
  print $ product' [1,2,3,4,5] -- 120

  -- 先頭2件を抽出
  print $ take' 2 [1,2,3,4,5]  --  [1,2]

  -- 先頭2件を除いたものを抽出
  print $ drop' 2 [1,2,3,4,5] -- [3,4,5]

  -- 逆順
  print $ reverse' [1,2,3,4,5]  -- [5,4,3,2,1]