Haskell 錯誤 - 無法將類型“[Int] -> String”與“[Char]”匹配； 預期類型：字符串； 實際類型：[Int] -> 字符串

Question

我正在嘗試在 Haskell 中運行 Hopscotch 練習，但在編譯時遇到此錯誤。 將不勝感激任何幫助。

我檢查了參數和輸出，它們似乎是正確的。

這是錯誤信息：

Histogram1.hs:11:16: error:
    * Couldn't match type `[Int] -> String' with `[Char]'
      Expected type: String
        Actual type: [Int] -> String
    * Probable cause: `(.)' is applied to too few arguments

這是哈斯克爾代碼：

module Histogram where

import qualified Data.Map as M 

--- Type Aliases
type Count = Int 
type Counts = M.Map Int Count
type Indices = [Int]  

histogram :: [Int] -> String
histogram xs = histogram' . toCounts 
  where 
    histogram' :: Counts -> String 
    histogram' xs = 
      let (maxs, xs') = trim xs 
      in if null maxs 
            then footer 
            else asterisks maxs ++ histogram' xs' 
  
    footer :: String 
    footer = unlines ["=========="
                      , "0123456789"
                      ]  


toCounts :: [Int] -> Counts 
toCounts = foldr insertOrAdjust M.empty 

asterisks :: [Int] -> String 
asterisks ns = map (\n -> if n `elem` ns then '*' else ' ') [0..9] ++ "\n"

Answer 1

問題出在這一行

histogram xs = histogram' . toCounts

它應該是

histogram xs = histogram' $ toCounts xs

或者

histogram = histogram' . toCounts

你寫的是函數組合，它產生[Int] -> String但是你已經聲明了另一個變量。