如何在 main 中使用用户输入调用函数？

Question

Libs.hs：

--TAKE INPUT OF USER CARD DETAILS AND SEPERATE EACH DIGIT AND INTO A LIST

getCard :: Integer -> [Integer]
getCard x
    | x <= 0 = []
    | otherwise = lst_numb : getCard pre_numb
      where
        (pre_numb, lst_numb) = x `divMod` 10

--TAKE SEPERATED DIGITS LIST AND REVERSE

cardNumber :: Integer -> [Integer]
cardNumber = reverse . getCard


--DOUBLE EVERY OTHER NUMBER STARTING FROM THE RIGHT (TO THE LEFT OF CHECK NUMBER)

doubleNumber :: [Integer] -> [Integer]
doubleNumber [] = []
--doubleNumber (x:y:xs) = 2*x : y : doubleNumber xs //This is the partially working function for Q2
doubleNumber (x:y:xs)
    | validFormat (x:y:xs) == True = 2*x : y : doubleNumber xs
    |otherwise = []


--CHECKS ISSUER ID IS VALID & CARD NUMBER IS 16 DIGITS LONG

validFormat :: [Integer] -> Bool
validFormat  x
    | x == [] = False
    | (head x == 3 || head x == 4 || head x == 5 || head x == 6) && length x == 16 = True
    | otherwise = False

--ADDITION OF ALL DIGITS INSIDE THE LIST
addNumbers :: [Integer] -> Integer
addNumbers xs = sum xs

--SUBTRACT NINE FROM DOUBBLE DIGIT NUMBERS

subDoubles :: [Integer] -> [Integer]
subDoubles [] = []
subDoubles (x:xs)
    | x > 9 = x - 9 : subDoubles xs
    | otherwise =  x : subDoubles xs

--COMBINED FUNCTION TO VALIDATE A CREDIT CARD NUMBER

validateCard :: Integer -> Bool
validateCard x = compute x `mod` 10 == 0
    where
      compute :: Integer -> Integer
      compute = addNumbers . subDoubles . doubleNumber . cardNumber

--DISPLAY USER MSG IF CARD IS VALID OR NOT
isValid card
  | card == True = "This is a valid credit card!"
  | otherwise = "This card is invalid"

Main.hs：

main :: IO ()
main = do
    putStrLn "Please enter a credit card number:"
    input <- getLine
    let card = read input :: Integer
    isValid . validateCard card

我有一个 Haskell 项目，可以根据用户输入验证信用卡号（如上面在 Libs.hs 中）：

我可以在 ghci 中使用带有数字的 validateCard 函数，它会产生真或假。我希望用户在 Main.hs 文件中输入一个信用卡号，并获得一个字符串来宣传他们的输入是有效还是无效，但是，我不知道我是否正确调用了 main.hs 中的 validateCard 函数。有人可以解释一下我的方法哪里出了问题吗？ 已经有一段时间了，所以任何建议或建议将不胜感激。提前谢谢！

编辑： 我上传的代码是我正在尝试的代码，但这是我尝试的初始方式，它无法正常工作

Answer 1

您在以下方面混淆了优先规则：

isValid . validateCard card

这会解析为

isValid . (validateCard card)

但这不起作用，因为validateCard card 不是函数。你需要写其中之一：

isValid . validateCard $ card
isValid $ validateCard card

Answer 2

Robin Zigmond 在他的评论中这么说，但你正在寻找getLine：

getLine :: IO string

来自main，你会这样使用它：

main = do
    input <- getLine