Catchable

A context in which exceptions can be caught and thrown.

This class places no other class constraints on F, but it should be the case that exceptions raised via fail are caught by the nearest surrounding attempt and returned as a Left. In addition to catching explicitly raised exceptions via fail, we expect that attempt catch ambient exceptions that might occur when 'evaluating' an F.

We can state the requirement that attempt catch all ambient exceptions by stipulating that for all total functions of the form g: forall A . F[Throwable \/ A] => B, g compose attempt is also total.