exception - Haskell 错误 : "Ambiguous type variable" in Exception. 捕获；需要类型签名，但是在哪里？

Question

我正在尝试将程序的错误处理转换为使用异常。我构建了一个 REPL 来控制我的模拟，同时我还处理错误:

> repl :: Sim -> IO Sim
> repl old_sim = do
>     new_sim <- E.catch (do line <- getLine
>                            commands <- parseCommand line
>                            runCommands commands old_sim)
>                        (\err -> do putStrLn . show $ err
>                                    return old_sim)
>     if alive new_sim
>        then repl new_sim
>        else return new_sim

parseCommands 或 runCommands 都可能失败 String，两者都会返回 IO Sim。

当我加载文件时，我收到这个巨大的错误:

Prelude> :l simple_sim
[1 of 1] Compiling Main             ( simple_sim.lhs, interpreted )

simple_sim.lhs:95:18:
    Ambiguous type variable `a0' in the constraints:
      (E.Exception a0) arising from a use of `E.catch'
                       at simple_sim.lhs:95:18-24
      (Show a0) arising from a use of `show' at simple_sim.lhs:100:49-52
    Probable fix: add a type signature that fixes these type variable(s)
    In a stmt of a 'do' expression:
        new_sim <- E.catch
                     (do { line <- getLine;
                           commands <- parseCommand line;
                           runCommands commands old_sim })
                     (\ err
                        -> do { putStrLn . show $ err;
                            return old_sim })
    In the expression:
      do { new_sim <- E.catch
                        (do { line <- getLine;
                              commands <- parseCommand line;
                              runCommands commands old_sim })
                        (\ err
                           -> do { putStrLn . show $ err;
                               .... });
           if alive new_sim then repl new_sim else return new_sim }
    In an equation for `repl':
        repl old_sim
          = do { new_sim <- E.catch
                              (do { line <- getLine;
                                    commands <- parseCommand line;
                                    .... })
                              (\ err -> do { ... });
                 if alive new_sim then repl new_sim else return new_sim }
Failed, modules loaded: none.

具体:

Probable fix: add a type signature that fixes these type variable(s)

我应该在哪里插入这个类型签名？

Answer 1

您需要在 err 上使用它来确定您要 try catch 哪些异常。