[int_19h]

How do you handle the situation where the code might need to throw (pre-existing) exceptions that don't share a useful base class?

[iainmerrick]

I don’t remember! Possibly that’s one of the cases where it doesn’t work out.

Of course, if you had proper sum types, that situation wouldn’t be a problem.

[gf000]

Java has proper sum types, but what one needs here is union types. They are not the same, sum types are labeled and are disjoint.

You want `MyException | ThirdPartyException` here, though.

[iainmerrick]

You're right, I had it backwards! Thanks for the correction.

Now I'm wondering, could that actually be all that's needed to rescue checked exceptions? Is there any language that has that combination of features?

[int_19h]

Back when Java didn't have lambdas, one of the more advanced lambda proposals (http://www.javac.info/closures-v06a.html) had this exact thing for this exact reason.

Unfortunately, this take on lambdas was deemed too complicated, and so we got the present system which doesn't really try to deal with this use case at all.

[gf000]

Well, scala has union types, but it doesn't do checked exceptions per say (but it does have a very advanced type system so similar structure can be easily encoded). I think checked exceptions is pretty rare, so I don't know.. probably some research language (but they often go the extra mile towards effect types)