Handling effects

After a component succeeds, it hands a result back to the step. That effect might be a computed value, a record to collect, a request for another action, or several outputs. Your step decides what to do with it.

Apply a result

Apply<E> takes ownership of one effect and applies it to a destination you choose. For example, the standard PushToStack protocol inserts typed values into operand storage. Call it after execute; execution alone won’t apply the result.

Apply has no error return. If handling a result involves validation or other fallible work, do that in your step or a helper and propagate the error there. You don’t need an Apply implementation for every application effect. A step can also store a result in another device or pass it to an application method.

Handle several effects

Effects<E> holds zero, one, or many outputs. You can build up a collection and iterate over its values in order:

use vihaco::Effects;

let effects = Effects::one(20_u64).append(22);
let results: Vec<_> = effects.into_iter().collect();
assert_eq!(results, vec![20, 22]);

In a step, handle each effect according to what it means for your machine. If handling one result produces more work, decide whether to finish that work before moving on or queue it for later. Vihaco leaves that choice to you.

Preserve failure behavior

A helper can change device state and then return an error. Earlier calls and stack insertions stay in place. Decide whether an error should stop processing, and make sure you handle results that matter to execution.

Next, see how these steps fit into running a program.