Haskell 问题：解析错误和空 main

Question

我必须弄清楚如何编写一个 Haskell 程序来执行以下操作：

1. 向用户询问容器的长度、宽度和高度
2. 询问最大允许质量
3. 计算并显示最大允许密度

在玩弄了一些代码并花了几个小时学习 LearnYouAHaskell 之后，我得出了这个结论：

main = do
 getLength

isNumber :: String -> Bool
isNumber line = if (null $ filter (`notElem` ['0'..'9'] ++ ['.']) line)
  then True
  else False

getInput :: IO Double
getInput = do
 line <- getLine
 if isNumber line
  then return (read line :: Double)
  else do
  putStrLn "This value is not a number, please try again!"
  getInput

getLength :: IO ()
getLength = do
 putStrLn "-----------------------"
 putStrLn "DensityCalc v1.0"
 putStrLn "-----------------------"
 putStrLn ""
 putStrLn "Please enter the container length to begin."
 putStrLn "Entering 0 will end the program."
 length <- getInput
 if length == 0
  then putStrLn "Goodbye!"
  else do
   putStrLn $ "Length: " ++ show length
   putStrLn "Now enter container width (0 to quit): "
   getWidth length

getWidth :: Double -> IO ()
getWidth length = do
 width <- getInput
 if width == 0
  then putStrLn "Goodbye!"
  else do
   putStrLn $ "Width: " ++ show width
   putStrLn "Now enter container height (0 to quit): "
   getHeight length width

getHeight :: Double -> Double -> IO ()
getHeight length width = do
 height <- getInput
 if height == 0
  then putStrLn "Goodbye!"
  else do
   putStrLn $ "Height: " ++ show height
   putStrLn "Finally, enter maximum allowed mass (0 to quit): "
   getMass length width height   

getMass :: Double -> Double -> Double -> IO ()
getMass length width height = do
 mass <- getInput
 if mass == 0
  then putStrLn "Goodbye!"
  else do
   putStrLn $ "Maximum mass: " ++ show mass
   putStrLn "Calculating density..."
   getDensity length width height mass

getDensity :: Double -> Double -> Double -> Double -> IO ()
getDensity length width height mass = do
 let density = mass * 100.0 / length * width * height
 let roundedDensity = round (density :: Double)
 let shownDensity = (fromIntegral roundedDensity) / 100.0
 putStrLn $ "Maximum allowed freight density: " ++ show shownDensity
 putStrLn "Thank you for using my tool! Goodbye!"

我的第一个问题是：编译时，它在第 65 行给了我一个“输入 =" 解析错误 -

density = mass * 100.0 / length * width * height

其次，如果我完全取出密度函数，它会编译 - 但主函数什么都不做（返回我输入的内容，没有别的） - screenshot

我应该在哪里查找错误？

[编辑]

第一个问题解决了，改了代码。第二个仍然存在。

[编辑2]

看来，为了开始，我需要在主窗口中手动输入“getLength”。我可以让它立即调用该函数吗？

Answer 1

您需要let 在do 中定义新的绑定。

Answer 2

发现我需要在使用程序调用函数时手动编写“getLength”，然后一切正常。

问题是 - 如果 getLength 在“main = do”子句中明确提到，为什么我必须这样做？