haskell中的NILL值

Question

我从用户那里获得输入(x),通过让y =(读取x):: Int将其转换为Int,然后我希望函数在用户什么都不给的情况下以特殊方式运行(空字符串).

-- In this place I would like to handle situation in which user
-- gave empty string as argument
-- this doesnt work :/
yearFilter [] y = True

--This works fine as far as y is integer
yearFilter x y  | x == (objectYear y) = True
                | otherwise = False

谢谢你的帮助,再见

Answer 1

也许你想要一个Maybe类型？如果用户输入空字符串,则函数返回Nothing; 否则它返回Just n,n用户输入的是什么？

userInt :: String -> Maybe Int
userInt [] = Nothing
userInt s  = Just $ read s

(我还没有编译这段代码.)

Answer 2

除非明确定义，否则不存在 NULL。您可以像这样检查空字符串。

readInput :: IO ()
readInput = do
   ln <- getLine
   if valid ln
      then -- whatever
      else -- whatever

valid x
    | null x                = False
    | not istJust convert x = False
    | otherwise             = True
where convert :: String -> Maybe Int
      convert = fmap fst $ listToMaybe  . reads $ "f"