Module 07 — Errors

Goal

Make failures across the language boundary useful, not opaque.

Rust side

Domain operations return Result<T, KernelError>:

pub enum KernelError {
    InvalidIdentifier(String),
    InvalidManifest(String),
    RuleEvaluation(String),
}

Python side

Those errors become typed exceptions:

from open_engineering_kernel import (
    Identifier,
    InvalidIdentifierError,
    InvalidManifestError,
    RuleEvaluationError,
)

try:
    Identifier.parse("not-valid")
except InvalidIdentifierError as e:
    print(e)   # clear message, catchable by type

What to avoid

Anti-pattern	Why it is bad
panic! crossing the boundary	Turns into an opaque crash
Returning only a string	Callers cannot distinguish error kinds
Swallowing errors in Rust	Hides failures from Python

Design rule

The boundary is part of the public API.

Error types and messages are therefore a deliberate product decision, not an afterthought.

Exercise

1. Trigger InvalidIdentifierError and InvalidManifestError from Python.
2. Confirm each can be caught by its specific exception class.
3. Read the message and decide whether a future learner would understand what went wrong.