[Haskell-cafe] IO and exceptions

[Ömer Sinan Ağacan]

Hi all,

I'm having some trouble with IO functions and exceptions.

When I call an IO function in Haskell, I feel very uncomfortable,
because you never know what kind of exceptions your function may
throw. There are a lot of cases where thrown exceptions are not
specified in the documentation. What's worse, there are cases where
it's impossible to catch thrown exceptions in a safe way.

Example to not knowing what exceptions may be thrown:
http://hackage.haskell.org/package/base-4.6.0.1/docs/Control-Concurrent-Chan.html
`readChan` blocks the thread until it reads something. But what
happens when channel is closed while one thread is blocked to read?
It's not mentioned in the documentation and it's not specified in
terms of types. (e.g. by using Either, Maybe or similar)

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.

I was wondering if I'm doing something wrong. I'm sure we can find a
lot of cases like these and I feel like libraries are not designed
very good for errors/exceptions.

So I'd like to get some ideas/suggestions about this. Am I doing
something wrong or did I misunderstand something? Should we improve
the libraries for better types/error handling? Am I only one here that
has problems with those functions?

Thanks.