function - 我需要编写一个haskell 函数来复制字符串中出现的每个字符，两者都是由用户给出的。

Question

到目前为止，这就是我所拥有的。我知道它与函数本身有关，因为我有 Char -> String -> String，但我需要用户输入它，所以 IO.我不太确定如何做到这一点。

dup :: Char -> String -> String
dup c [] = []
dup c (x:xs)
  | c == x    =  x:x:dup c xs
  | otherwise =  x:dup c xs

main = do
  putStrLn "Enter a sentence."
  sentence <- getLine
  putStrLn "Enter a single letter that is in your sentence, to duplicate."
  letter <- getLine
  let x = dup 'letter''sentence'
   putStrLn x

Answer 1

很近。哪里getLine给出 String , 或 [Char] ，你只想要一个 Char .你可以这样做:

main = do
  putStrLn "Enter a sentence."
  sentence <- getLine
  putStrLn "Enter a single letter that is in your sentence, to duplicate."
  -- Get first character only
  letter:_ <- getLine
  let x = dup letter sentence
  putStrLn x

对此有一些警告，因为模式匹配可能会失败，但它会起作用，并且与您编写的内容非常接近。