[Haskell-cafe] Exceptions

Marc Weber marco-oweber at gmx.de
Wed Sep 17 20:51:48 EDT 2008

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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