Tryst::ErrorHandler

An on_error: handler - called with the exception instead of applying a policy. Only a handler lets a repeating timer keep running; both policies stop it, and differ just in whether the exception surfaces.

A policy and a handler are separate overloads rather than one ErrorPolicy | Proc parameter, and that is load-bearing. A proc literal whose body always raises has type Proc(Exception, NoReturn), which is accepted wherever Proc(Exception, Nil) is expected but is NOT an is_a?(Proc(Exception, Nil)) at runtime. Put one in a union and every branch testing for the proc silently fails to match - with an exhaustive case ... in that means falling through to unreachable code and hanging, not an error. Keeping the two apart means nothing ever has to type-test a Proc.