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
