John, have you seen this? http://haskell.galois.com/cgi-bin/haskell-prime/trac.cgi/wiki/Extensible Exceptions Cheers, Simon On 07 April 2006 15:34, John Goerzen wrote:
Hello,
One thing that bugs me about Haskell is that exceptions are not extensible.
I don't know how to craft a good solution, but perhaps if I explain the problem well, someone would come up with one.
In a language such as Python or Java, and exception is an object.
Let's consider Python for a quick example. Python has an IOError exception. So if I want to write a handler that deals with IOErrors, that's easy enough.
Now maybe I want to do something like report socket errors specially. I could define a SocketError class that subclasses IOError. I could take this further, and define a URLError that subclasses SocketError.
Now the beauty of it is that I can:
* Have a handler that catches URLErrors only and does nothing special with SocketErrors or IOErrors.
* Have a handler -- perhaps not even mine -- that catches and works with IOErrors. Since SocketError and URLError are descendants of IOError, that handler will *automatically* work if I raise a SocketError or a URLError.
I can see no such mechanism in Haskell. Haskell's I/O exceptions have a certain defined set of errors that they can report, and I can't subclass them and make them more specific for my purposes if I want. Ditto for all the others.
The Dynamic exception support is necessary and good to have, but it also under-documented and can be complex. And of course, they still suffer from the same lack of extensibility
Are there any suggestions on how we might improve this situation in Haskell?
-- John