小编use*_*063的帖子

defaultFileName :: [Char]
defaultFileName = "Test.log"

defaultSearchName :: String
defaultSearchName = "xyz"

可以编译此代码：

a3 :: Int -> [[Char]] -> IO [Char]
a3 index arg = 
  if null arg
    then do
      a <- putStrLn "No parameters have been passed."
      a <- putStrLn $ "1 Default search string: " ++ defaultSearchName
      a <- putStrLn ("2 Default file name: " ++ defaultFileName)
      return defaultFileName
    else return (arg!!index)

一旦我添加了一个额外的 IF-THEN-ELSE，我就不能再编译它了

a3 :: Int -> [[Char]] -> IO [Char]
a3 index arg = 
  if null …