組み合わせ

programming haskell

前回の改良バージョンを眺めていると組み合わせも簡単に作れることに気付く．
takeOne のときに 前半を捨ててしまえばよさそうだ．



takeOne' :: [a] -> [(a, [a])]

takeOne' [] = []

takeOne' (x:xs) = (x, xs) : takeOne' xs
comb :: Int -> [a] -> a

comb _ [] = [[]]

comb 1 xs = do x <- xs

               return [x]

comb n xs = do (x, xs') <- takeOne' xs

               guard $ length xs' /= 0

               xs'' <- comb (n - 1) xs'

               return $ x : xs''
comb のときに長さが足りなくなったら失敗させる (下から3行目)