异常类型错误

Question

我正在学习如何在Haskell中使用异常。
当试图复制在前奏this简单的例子，我得到：

GHCi, version 7.10.2: http://www.haskell.org/ghc/ :? for help 
Prelude> :m Control.Exception 
Prelude Control.Exception> let x = 5 `div` 0 
Prelude Control.Exception> let y = 5 `div` 1 
Prelude Control.Exception> print x 
*** Exception: divide by zero 
Prelude Control.Exception> print y 
5 
Prelude Control.Exception> try (print x) 

<interactive>:16:1: 
    No instance for (Show (IO (Either e0()))) 
     arising from a use of `print' 
    In a stmt of an interactive GHCi command: print it 
Prelude Control.Exception> 

为什么我得到没有实例错误上try(print x)，当我以前有一个例外？

Answer 1

问题是，哈斯克尔/ GHCI不知道的e0的类型，所以你必须将其标注为：

try (print x) :: IO (Either ArithException()) 

的原因是，在编译时有相当多的可能实例（不同的例外）：see here for a description - 和GHCI不能选择

你可以得到GHCI告诉你，所以如果你看起来有点更深的表情（或多或少不直接强迫GHCI到show吧）：

Prelude Control.Exception> e <- try (print x) 

<interactive>:5:6: 
    No instance for (Exception e0) arising from a use of `try' 
    The type variable `e0' is ambiguous 
    Note: there are several potential instances: 
     instance Exception NestedAtomically 
     -- Defined in `Control.Exception.Base' 
     instance Exception NoMethodError 
     -- Defined in `Control.Exception.Base' 
     instance Exception NonTermination 
     -- Defined in `Control.Exception.Base' 
     ...plus 7 others 
    In the first argument of `GHC.GHCi.ghciStepIO :: 
           IO a_a18N -> IO a_a18N', namely 
     `try (print x)' 
    In a stmt of an interactive GHCi command: 
     e <- GHC.GHCi.ghciStepIO :: IO a_a18N -> IO a_a18N (try (print x)) 

---

当然

你不必猜测正确的异常（如我做了与ArithException） - 而不是你可以使用SomeException到抓所有太：

Prelude Control.Exception> try (print x) :: IO (Either SomeException()) 
Left divide by zero