The downside of doing that is that you can't do nice simple things like
(when (find 1 '(2 3 4)) (prn "Found 1"))
You would have to use match instead, but I agree there will always be cases where you do need that option.
I think defcon and match are a great idea, although I don't think you need all that power for what you're doing. Returning a uniq that indicates failure would be just as effective. defcon would be good as a standard way to create new types, and because they'd all work with match you'd gain a lot of power by following that convention. This kind of destructuring is one of ML's best features and I was temped to implement it for Arc myself.
