Question

我必须弄清楚如何编写一个执行以下操作的Haskell程序：

询问用户容器长度，宽度和高度
询问最大允许质量
计算并显示最大允许密度

在学习了LearnYouAHaskell的代码和几个高质量的小时后，我来到这里：

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：在编译时，它会在输入中给出一个＆＃34;解析错误=＆＃34;在第65行 -

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，而不是其他内容） - screenshot

我应该在哪里寻找错误？

[编辑]

第一个问题解决了，改了代码。第二个仍然存在。

[EDIT2]

似乎，为了开始，我需要手动输入＆＃34; getLength＆＃34;在主窗口中。我可以立即调用它吗？

Answer 1

您需要let在do内定义新的绑定。

Answer 2

想出我在使用程序调用函数时需要手动编写“getLength”，然后一切正常。

问题是 - 如果在“main = do”子句中明确提到了getLength，为什么还要这样做呢？

Haskell问题：解析错误和空主

2 个答案: