Haskell IO 返回无法匹配类型 ‘[Char]’ 与 ‘Char’ 预期类型 : String Actual type: [String]

Question

嗨，我正在尝试创建一个用户界面，用户可以在其中选择 mainMenu 来测试该功能。

data Place =  Place String Coord [Int] 
             deriving (Ord,Eq,Show,Read)

data Coord =  Cord Double Double
             deriving (Ord ,Eq ,Show ,Read)


placeName :: [Place] -> [String]
placeName []       = [""]
placeName plcs   = [s | (Place s x(n:ns))<-plcs] 




main :: IO ()
main = do putStrLn "Rainfall Map Without A Map"
          placeFile <- readFile "place.txt"
          let placeData = read placeFile
          putStrLn ("\nFile Loaded "++ show(length placeData)++ "places")
          mainMenu placeData

mainMenu ::  [Place] -> IO()
mainMenu plcs = do 
 putStrLn(" 1.  return the name of all places")
 putStrLn(" 2.  Display the average rain figures of a place")
 putStrLn(" 3.  Display places with formatted String")
 putStrLn(" 4.  Return the place that have zero raingfall")
 putStrLn(" 5.  Replace a place with new place")
 putStrLn(" 6.  Save and Exit")
 putStr("Please insert a number: ")
 choose<- getLine
 numberChoose choose plcs

numberChoose :: String -> [Place]->IO()
numberChoose "1" plcs  = first plcs 
numberChoose "2" plcs  = second plcs
numberChoose "3" plcs  = third plcs
numberChoose "4" plcs  = fourth plcs
numberChoose "5" plcs  = fifth plcs
numberChoose "6" plcs  = sixth plcs
numberChoose _ plcs = do 
           putStrLn ("Error!:-- Wrong Input")
           mainMenu plcs


first :: [Place]->IO()
first plcs  = do
   putStrLn $  placeName plcs
   mainMenu  plcs 

所以我的 placeName 函数仅用于从 Place 返回字符串列表，而我的第一个函数仅用于在用户插入相应的 num 时返回字符串列表。我假设我的 mainMenu 显示的数据类型与我的第一个函数相同。它给了我错误:

    • Couldn't match type ‘[Char]’ with ‘Char’
      Expected type: String
        Actual type: [String]
    • In the second argument of ‘($)’, namely ‘placeName plcs’
      In a stmt of a 'do' block: putStrLn $ placeName plcs
      In the expression:
        do putStrLn $ placeName plcs
           mainMenu plcs
    |
153 |    putStrLn $  placeName plcs     |                ^^^^^^^^^^^^^^

Answer 1

putStrLn有类型 putStrLn :: String -> IO () ，所以它打印一个字符串，而不是一个字符串列表。

您可以使用 mapM_ :: (Foldable f, Monad m) -> (a -> m b) -> f a -> m () 对每个元素执行一元函数，所以:

first :: [Place] -> IO ()
first plcs  = do
   mapM_ putStrLn (placeName plcs)
   mainMenu  plcs

或者您可以使用 unlines :: [String] -> String 将字符串列表转换为由新行分隔的字符串:

first :: [Place] -> IO ()
first plcs  = do
   putStrLn (unlines (placeName plcs))
   mainMenu  plcs