design-patterns - Haskell 错误的 IO 输出

Question

这就是我所拥有的，我想要这样的输出

*Main> sumThree
Please enter three integers:
1
2
3
Their sum is: 6
(1,2,3)

然而，我不断得到，没有总和

*Main> sumThree
Please enter three integers:
1
2
3
Their sum is:
(1,2,3)

**summ :: Integer -> Integer -> Integer -> IO Integer
summ x1 x2 x3 = return(x1+x2+x3)
sumThree :: IO (Integer, Integer, Integer)
sumThree = do putStr "Please enter tree integers:"
              x1 <- getInteger
              x2 <- getInteger
              x3 <- getInteger
              putStr "Their Sum is: "
              summ x1 x2 x3
              return(x1,x2,x3)**

Answer 1

您必须实际打印 summ 的结果.

不要return在您的 summ函数，所以函数的结果类型是 Integer而不是 IO Integer .然后，做

putStr (show (summ x1 x2 x3))

而不仅仅是 summ x1 x2 x3 .

此外，您确实了解 sum内置(Prelude)功能吧？它需要一个数组 Num的，所以你可以做 putStr (show (sum [x1, x2, x3])) .