Revisit exception mechanism

[benl]

We eliminated the effect masking operator from the type system due to the poisoning problem. Perhaps we can do something with Benton + Buchlovsky's "Semantics of an effect analysis for exceptions" paper.

I remember seeing a ML paper on this saying there was a better way than try-catch. What was that?

Want typed exceptions, so we can thrown an object of arbitrary (user defined) type.