[Haskell-cafe] IO and exceptions

[Alberto G. Corona]

In the other side, this package does exactly what you want (I think).


2014-11-09 14:11 GMT+01:00 Roman Cheplyaka <roma at ro-che.info>:

> Let's say you knew what exceptions could be thrown. What would you do
> differently then?
>
> Typically, if it's a command-line app and something unexpected happens,
> you simply present the error to the user. That's what exceptions do
> already without any effort from you.
>
> If it's a server app, you catch exceptions at a certain point, log them,
> maybe give some feedback to the client and proceed with the other
> requests. In order to do that, you don't really need to know the set of
> all possible exceptions.
>
> > `readChan` blocks the thread until it reads something. But what
> > happens when channel is closed while one thread is blocked to read?
>
> What do you mean by "channel is closed"? Channels don't get closed;
> there's no function to do that.
>
> It's possible that your blocked thread has the only remaining reference
> to the channel; in that case, the deadlock may (or may not) be detected,
> and an exception will be thrown in the former case.
>
> > Example to not being able to catch the exception:
> >
> http://hackage.haskell.org/package/network-2.6.0.2/docs/Network-Socket.html
> > `recvFrom` blocks the thread until it reads something. However, what
> > happens if socket is closed while `recvFrom` is blocking to read?
> > Similar to `readChan`, we can't know this. What's worse is this:
> > sockets have a `SO_RCVTIMEO` option, a timeout value for `recv_from`
> > calls. What happens when we set that using `setSocketOption` in the
> > same module and call `recvFrom` is unknown. This is really horrible,
> > because a timeout exception is really something that you'd like to
> > catch. Example: I have a thread that listens from a socket and handles
> > some other events. I want to stop listening the socket after every
> > second and handle events, then return listening the socket. But I
> > can't know what it throws when timeout happens, so I have to catch all
> > exceptions, which is horrible for a lot of reasons.
>
> It sounds like you're trying to write C-style in Haskell, which is
> probably not the best way to do it.
>
> More Haskelly way would be to:
>
> 1. Use Network instead of Network.Socket and take advantage of GHC's IO
> manager. That means that you'll be using something like epoll instead of
> recvfrom under the hood.
> 2. Instead of manually multiplexing several things into one thread
> (listening on the socket and "handling some other events"), do that in
> different threads.
> 3. If you do need a timeout (not for multiplexing, but for a genuine
> timeout), use System.Timeout.
>
>
> Roman
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-- 
Alberto.
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