Haskell 如何使这样的函数'shouldBeRight:：a b->；IO b`？

为了得到“正确”的值或“公正”的值，我尝试做如下两个函数。但是不能编译，为什么

shouldBeRight :: (Show a) => Either a b -> IO b
shouldBeRight (Left a) =
  throwIO $ concat ["should be Right value but wasn't, because:", show a]
shouldBeRight (Right b) = return b

shouldBeJust :: Maybe a -> IO a
shouldBeJust Nothing = throwIO "should be Just value but was Nothing"
shouldBeJust (Just a) = return a

以下是错误：

    • Ambiguous type variable ‘e0’ arising from a use of ‘throwIO’
      prevents the constraint ‘(GHC.Exception.Exception
                                  e0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘e0’ should be.
      These potential instances exist:
        30 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the expression: throwIO "should be Just value but is nothing"
      In an equation for ‘shouldBeJust’:
          shouldBeJust Nothing
            = throwIO "should be Just value but is nothing"
   |
23 | shouldBeJust Nothing = throwIO "should be Just value but is nothing"
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
...

    • Ambiguous type variable ‘e0’ arising from the literal ‘"should be Just value but is nothing"’
      prevents the constraint ‘(Data.String.IsString
                                  e0)’ from being solved.
      Probable fix: use a type annotation to specify what ‘e0’ should be.
      These potential instances exist:
        instance a ~ Char => Data.String.IsString [a]
          -- Defined in ‘Data.String’
        ...plus 13 instances involving out-of-scope types
        (use -fprint-potential-instances to see them all)
    • In the first argument of ‘throwIO’, namely
        ‘"should be Just value but is nothing"’
      In the expression: throwIO "should be Just value but is nothing"
      In an equation for ‘shouldBeJust’:
          shouldBeJust Nothing
            = throwIO "should be Just value but is nothing"
   |
23 | shouldBeJust Nothing = throwIO "should be Just value but is nothing"
   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

---

编译器不知道您试图通过

throwIO

执行的异常类型

从错误消息判断，您可能启用了

-XOverloadedStrings

，这意味着字符串文本（例如

“abcd”

）没有确定的类型，但可以采用预期的任何类型，只要有

IsString

实例。最常用的字符串类型是

string

和

Text

，但也有其他类型，您也可以自己制作

另一方面，函数

：：Exception e=>e->IO a

也没有指定任何特定的类型：它接受任何类型

e

，只要它有一个

Exception

实例

所以编译器不知道选择哪种类型。挑选任何东西都没有依据

解决这个问题的“正常”方法是创建您自己的异常类型，并为其提供一个

异常

实例，如下所示：

data MyConversionException = NotJust | NotRight String deriving (Typeable, Show)

instance Exception MyConversionException

然后扔掉它：

shouldBeRight (Left a) = throwIO $ NotRight (show a)

...

shouldBeJust Nothing = throwIO NotJust

如果确实希望异常发生时显示该特定文本，则始终可以为其提供一个自定义

Show

实例：

instance Show MyConversionException where
    show NotJust = "should be Just value but was Nothing"
    show (NotRight leftValue) = "should be Right value but wasn't, because:" ++ leftValue

或者，如果您只是想要一个quick-n-dirty解决方案，您可以使用该函数，该函数生成运行时异常，非常类似于

throwIO

，但其类型为纯，因此也可以在IO外部使用：

shouldBeJust Nothing = error "should be Just value but was Nothing"

---

如果说

throwIO

的参数需要是

Exception

类的一个实例，而

String

似乎不是，那么这似乎是一种相当混乱的说法。或者至少《代码》中的throwIO建议了这样的解释。我明白了。使用

error

替代工作为什么在

IO

环境中提到

error

，在这里有一个更干净的机制可用？@dfeuer有两个原因：（1）从代码的其余部分判断，OP很可能在寻找一个快速修复方法，（2）评论中已经建议了

error

，而OP也选择了这样做。