Question

我在摆弄IO，但不理解以下错误：

* Ambiguous type variable `a0' arising from a use of `readLine'
      prevents the constraint `(Console a0)' from being solved.
      Probable fix: use a type annotation to specify what `a0' should be.
      These potential instance exist:
        instance Console Int -- Defined at Tclass.hs:8:14
    * In the second argument of `(+)', namely `readLine'
      In the second argument of `($)', namely `(2 + readLine)'
      In the expression: putStrLn . show $ (2 + readLine)
   |
17 |     useInt =putStrLn . show $ (2+readLine)

代码

module Tclass where
    import System.Environment

    class Console a where
        writeLine::a->IO()
        readLine::IO a

    instance Console Int where
        writeLine= putStrLn . show 

        readLine = do
            a <- getLine
            let b= (read  a)::Int
            return b

    useInt::IO()
    useInt =putStrLn . show $ (2+readLine)

PS 我不知道编译器是否应该推断readLine的实例类型并在2方法中使用useInt进行添加？

Answer 1

2在Haskell中不仅是Int，而且是任何数字类型，包括Float,Double,Integer,...。其类型为Num a => a －一种适合每种数字类型的多态类型。

因此，您可以改用(2::Int)。然后您会发现(2::Int) + readLine是类型错误，因为readLine :: Int是错误的，所以我们只会得到readLine :: IO Int。

您可以尝试使用此方法

useInt :: IO ()
useInt = do
   i <- readLine
   putStrLn . show (2 + i :: Int)

防止约束的模糊类型变量

1 个答案: