Simon Marlow simonmar at microsoft.com
Fri Apr 7 10:49:40 EDT 2006

John, have you seen this?



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

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