[Haskell-cafe] Exceptions

[Ryan Ingram]

Better is this:

data MalformedAddressException = MalformedAddressException String
deriving (Show, Typeable)

throwDynIO x = throwIO (DynException $ toDyn x)

-- in inet_error
... throwDynIO (MalformedAddressException "blah blah") ...

-- in HAppS-Server
... Exception.catchDyn (inet_addr uri) (\(MalformedAddressException s) -> ...)

  -- ryan

On Wed, Sep 17, 2008 at 5:51 PM, Marc Weber <marco-oweber at gmx.de> wrote:
> On Sun, Jul 27, 2008 at 07:23:14PM +0200, Adrian Neumann wrote:
>>  Hello,
>>
>>  I think it'd be nice if the compiler could warn me if there are any
>>  exceptions which I'm not catching, similar to checked exceptions in Java.
>>  Does anyone know of a possibility to do that in Haskell?
>
> He, I have found a use case for your request:
> from network
>
> inet_addr :: String -> IO HostAddress
> inet_addr ipstr = do
>   withCString ipstr $ \str -> do
>   had <- c_inet_addr str
>   if had == -1
>    then ioError (userError ("inet_addr: Malformed address: " ++ ipstr))
>    else return had  -- network byte order
>
>
> from HAppS-Server:
>
> host <- Exception.catch (inet_addr uri)    -- handles ascii IP numbers
>       (\_ -> getHostByName uri >>= \host ->
>            case hostAddresses host of
>                [] -> return (error "no addresses in host entry")
>                (h:_) -> return h)
>
> Very bad because this catches Exceptions thrown by trowTo as well,
> doesn't it?
>
> On the other hand just catching the UserError can be useless if the
> maintainers decide to throw a custom Exception in the future (which can
> and should be done in the future when extensible exceptions are standard?)
>
> In this case I would miss this update and miss to update the code. If we
> could only catch exceptions.
> Using Either would be another choice here. But it would lead to much
> more code.
>
> Anyway It think using Either is better because it can't lead to code as
> shown above.
>
> Another nice use case for Exceptions are timouts as implemented by HAppS
> as well.
> However I must conclude that a function call including the code above
> can just absorb my exception and rethrow another one (or in a worse case
> continue?) So maybe I have to change the TimOut code to do a
> forever (throwTo threadId TimOutException) to make sure it quits as fast
> as possible? This could lead to different trouble.
>
> So I think using Either is the best option although there is some more
> code to write.
>
> Marc Weber
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