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== Protocol-definition Language (PDL) ==
A protocol is between a ''parent'' actor and a ''child'' actor. Child actors are not trusted, and if there is any evidence the child is misbehaving, it is terminated. Code utilizing the child actor therefore will never see type or protocol errors; if they occur, the child is killed. The parent actor must, however, carefully handle errors. A protocol is specified with respect to the parent; that is, messages the parent is allowed to receive are exactly the messages a child is allowed to sent, and ''vice versa''.
A protocol consists of declarations of ''messages'' and specifications of ''state machine transitions''. (This ties us to state-machine semantics.) The message declarations are essentially type declarations for the transport layer, and the state machine transitions capture the semantics of the protocol itself.
'''TODO''': there are more things we can integrate into the transition grammar, but concrete use cases are necessary.
== Error Handling ==
Errors include
* Message type errors: bad data sent
* Protocol errors: e.g., wrong message, wrong state
* Transport errors: e.g., nesting asynchronicity within synchronicity
* Security errors: e.g., trying to send privileged info to plugins. '''TODO''': this is not officially part of the design yet
Error handling is different for parents and children. Child actors are not trusted, and if the parent detects the least whiff of malfeasance, the parent will invoke <code>child.KILLITWITHFIRE()</code>. Code utilizing the child actor therefore will never see type or protocol errors; if they occur, the child is killed.
If the child detects a parent error, there's not much it can do. The child will execute <code>self.SEPPUKU()</code>, but we may wish to have it first attempt to notify the parent of the error.
The parent actor must, however, carefully handle and recover from errors.
'''TODO''' more details
== Implementation ==
