[Haskell-cafe] Problem about exception.

[Magicloud Magiclouds]

On Thu, Jan 27, 2011 at 11:00 AM, Magicloud Magiclouds
<magicloud.magiclouds at gmail.com> wrote:
> On Thu, Jan 27, 2011 at 10:48 AM, Brandon S Allbery KF8NH
> <allbery.b at gmail.com> wrote:
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>> On 1/26/11 21:10 , Magicloud Magiclouds wrote:
>>> Hi,
>>>   Consider such a case: I'm making a client program. There is a
>>> process, client and server exchange some information in a strict
>>> order. So I do (persudo code):
>>> exchange = do
>>>   sendHello
>>>   readMsg >>= expect hello processHelloReply
>>>   sendWhatsyourname
>>>   readMsg >>= expect name processNameReply
>>>
>>>   And expect is something like:
>>> expect c f (cmd, msg) =
>>>   if c == cmd then
>>>     f msg
>>>   else
>>>     fail "unexpected"
>>>
>>>   This is OK until this situation: The server may send some special
>>> command, for example DEBUG, DISCONNECT. This breaks the process above.
>>> So I think I could throw the unexpected command to outer function to
>>> handle. Something like:
>>> main = do
>>>   connect
>>>   catch exchange $ \e -> do
>>>     case e of
>>>       UnexpectedCMD DEBUG -> -- ignore process
>>>       UnexpectedCMD DISCONNECT -> -- disconnect process
>>>       _ -> -- something really wrong
>>>
>>>   Well, with Control.Exception, I do not know how to make this done.
>>
>> It looks to me like the very example in the Control.Exception documentation
>> will do this, with some renaming.
>>
>>> -- many languages call this a "control exception"; think break/next
>>> -- etc.  It's an exception used internally to modify control flow.
>>> data ControlException = CEDebug | CEDisconnect
>>>      deriving (Show, Typeable)
>>>
>>> instance Exception ControlException
>>
>> So now you can trap your ControlException above, or anything else is
>> presumably a true exception.
>>
>>
>> - --
>> brandon s. allbery     [linux,solaris,freebsd,perl]      allbery at kf8nh.com
>> system administrator  [openafs,heimdal,too many hats]  allbery at ece.cmu.edu
>> electrical and computer engineering, carnegie mellon university      KF8NH
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>
> This is one way. But so the outer function could not know what
> happened in "really wrong" situation.
>
> --
> 竹密岂妨流水过
> 山高哪阻野云飞
>

Sorry, in last mail, I meant, in really wrong situation, there might
be other tens of unexpected command, I'd like to wrap them in one
exception, other than making tens of exceptions.

-- 
竹密岂妨流水过
山高哪阻野云飞