Using Messages With vihaco

A message is owned, runtime-supplied input for one instruction execution. It is separate from the instruction payload so a source program can name an operation while the composite supplies current machine state, timing data, capabilities, or values from another component.

Message types are ordinary Rust types. Implement the marker when the type is a meaningful runtime message:

use vihaco::Message;

#[derive(Debug)]
struct BinaryOperands { lhs: i64, rhs: i64 }
impl Message for BinaryOperands {}

The component declares the message through its Execute<I> implementation; the composite resolves it through one of the route clauses.

The three message sources

message none;                 // passes NoMessage
message from operand_stack;   // calls Supply<M>
message with resolve_message; // calls a composite method

message from field is useful when a reusable component already knows how to produce the message. message with method is the right boundary when several fields or machine policy must be combined:

impl Calculator {
    fn resolve_add(
        &mut self,
        _instruction: &calculator::instruction::Add,
    ) -> eyre::Result<BinaryOperands> {
        Ok(BinaryOperands { lhs: 1, rhs: 2 })
    }
}

The resolver returns an owned value. That matters for parked operations: the component must not retain a borrow into the composite while waiting for a completion.

Message, instruction, and effect

• The instruction is the runtime operation selected by a composite route.
• The message is resolved input for this execution attempt.
• The effect is owned output returned in StepResult.

Use instruction fields for values that are part of the encoded/runtime operation. Use messages for values supplied by the current machine state. Use effects for state changes or events that the parent must observe or route.

message = ... on the old component attribute is not part of the current API. The message contract belongs to Execute<I>, and its source belongs to the composite route.

Continue with Defining a Composite.