问Haskell错误:Exception.catch中的“类型变量不明确”；需要类型签名，但在哪里？

EN

> 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

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.

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