非穷举模式匹配只是因为我遗漏了`否则=`？(Non-exhaustive pattern matches only because I left off `otherwise =`? [duplicate])

 
 
 这个问题在这里已经有了答案： 
 
 
 
  
 GHC为什么抱怨非穷举模式？ 3个答案 
 
 
 
 
 我在Haskell中编写了一个简单的程序，它可以在Rust编程语言手册中描述玩猜谜游戏 ： 
 
 
 
 以下是它的工作原理：程序将生成一个1到100之间的随机整数。然后它会提示玩家输入猜测。 输入猜测后，它会指示猜测是否过低或过高。 如果猜测是正确的，游戏将打印祝贺并退出。 
 
 
 这是我写的： 
 
import Control.Monad (when)
import System.Random (randomRIO)

-- | Check if the guess is correct, otherwise provide a hint
respond :: Int -> Int -> String
respond correct guess
  | guess > correct = "Smaller than " ++ show guess
  | guess < correct = "Larger than " ++ show guess
  | guess == correct = "Correct! " ++ show correct

-- | Main game loop; prompt for input and repeat until guessed correctly
play :: Int -> IO ()
play x = do
  putStr "Guess: "
  guess <- read <$> getLine
  putStrLn $ respond x guess
  when (guess /= x) $ play x

-- | Start the game with a random number between 1 and 100
main :: IO ()
main = play =<< randomRIO (1, 100)
 
 代码有效，但是GHC给了我一个警告： "Pattern match(es) are non exhaustive. In an equation for 'respond': Patterns not matched: _ _" 
 
 我将这两个下划线表示我作为respond函数参数的两个Ints 。 我不明白的是我没有涉及的情况。 这些不是Maybe Int或任何特殊的 - 函数需要两个有效的Ints ，所以我只需要处理整数 - 我不认为有任何数字不能被视为大于，小于或等于另一个？ 
 
 这只是GHC，假设我没有覆盖所有的情况，因为我没有添加最终的otherwise =后卫？ 尽管它在逻辑上涵盖了所有情况。 
 
 
 另外，如果你有关于如何编写更多地道Haskell的技巧，我会很感激他们。 我仍然在学习基础知识。 

 
 
This question already has an answer here:
 
 
 
  
 Why is GHC complaining about non-exhaustive patterns?  3 answers  
 
 
 
 
I wrote a simple program in Haskell that plays the guessing game described in The Rust Programming Language book:
 
 
 
Here’s how it works: the program will generate a random integer between 1 and 100. It will then prompt the player to enter a guess. After entering a guess, it will indicate whether the guess is too low or too high. If the guess is correct, the game will print congratulations and exit.
 
 
Here is what I wrote:
 
import Control.Monad (when)
import System.Random (randomRIO)

-- | Check if the guess is correct, otherwise provide a hint
respond :: Int -> Int -> String
respond correct guess
  | guess > correct = "Smaller than " ++ show guess
  | guess < correct = "Larger than " ++ show guess
  | guess == correct = "Correct! " ++ show correct

-- | Main game loop; prompt for input and repeat until guessed correctly
play :: Int -> IO ()
play x = do
  putStr "Guess: "
  guess <- read <$> getLine
  putStrLn $ respond x guess
  when (guess /= x) $ play x

-- | Start the game with a random number between 1 and 100
main :: IO ()
main = play =<< randomRIO (1, 100)
 
The code works, but GHC gives me a warning that "Pattern match(es) are non exhaustive. In an equation for 'respond': Patterns not matched: _ _"
 
I take those two underscores to represent the two Ints I have as arguments to the respond function. What I don't understand is which case I haven't covered. Those aren't Maybe Ints or anything special — the function requires two valid Ints, so I only need to deal with integers — and I don't think there is any number that cannot be deemed greater than, less than, or equal to another?
 
Is this just GHC assuming I have not covered all cases because I didn't add a final otherwise = guard? Even though it logically covers all cases.
 
 
Also, if you have any tips on how to write more idiomatic Haskell, I'd appreciate them. I'm still learning the basics.

原文：https://stackoverflow.com/questions/46354999