Exceptions

• try handle =>
  Execute the expression . If is an exception literal, then if the exception is raised and not caught during the execution of , expression is evaluated. If is not a literal exception, it is viewed as a variable name to which any exception raised during the evaluation of will be bound, for an ensuing evaluation of .

  Using the variable-binding form of the try...handle block allows any PLAN exception to be caught. Therefore a simple error-reporting system may be set up. For example:

  fun exnreport(h:host,e:exn):unit =
    print("I raised ",e," on ",h)
  
  fun main(...) =
    try
      ...
    handle e => 
      OnRemote(exnreport(thisHost(),e),home,getRB(),defaultRoute)
  

• raise
  Raise the exception . Evaluate the handling expression of the tightest lexically-enclosing try...handle construct. If there is no such enclosing construct, the exception will cause the the termination of the program and the deletion of the packet.

Note that service calls and language constructs may raise exceptions which can be caught with try...handle blocks. As with uncaught exceptions raiseed by the PLAN program, these exceptions will cause the termination of the program and the deletion of the packet. The language-level exceptions are:

• ServiceNotPresent is raised when a service call is attempted for which the service is not resident on the active node. Note that because service calls are syntactically identical to function calls, the evaluator assumes that any unbound function name in an application is a service, thus raising this exception.
• DivByZero is raised upon an attempted division by zero.
• Hd is raised when hd is applied to an empty list.
• Tl is raised when tl is applied to an empty list.
• NotEnoughRB is raised when the current packet runs out of resource bound, or if it attempts to allocate to a child packet more resource bound than it has available.
• HostNotLocal is raised when a router is asked to forward a packet to a node which is not one of its link-layer connections.
• NoRouteEntry is raised when the invoked routing function does know of a route for the specified destination node.