On Mon, 2009-12-07 at 20:31 +0100, Henning Thielemann wrote:
Somehow I missed this thread. I want to say that I have implemented a general way to add the information of an exception to the "end" of an arbitrary data structure.
http://hackage.haskell.org/packages/archive/explicit-exception/0.1.4/doc/htm...
Duncan already mentioned it:
data Exceptional e a = Exceptional { exception :: Maybe e result :: a }
However it's pretty clear from the structure of this type that it cannot cope with lazy error handling in sequences. If you try it you'll find you cannot do it without space leaks.
Actually you spotted the space leak - but how is it pretty clear, that it has the space leak?
Yes you're right, it's not that clear. First time I saw the type my intuition was that would have a space leak but I had to do some work to demonstrate it. I was misremembering, it's only clear in retrospect :-)
I also agree with you, that the possibility to access the result directly, and thus easily ignore the exception is bad. I thought it could be improved by removing the field accessors and instead provide the function:
switch :: (Maybe e -> c) -> (a -> c) -> Exceptional e a -> c
True. As a general approach though, I think I rather like just making up extra algebraic types and handling them with their natural folds and unfolds. It's very much in the pure lazy FP tradition. Duncan