Error Handling — Cause, Exit, and Recovery

Part II gave you the full dependency injection story. Part III is about what happens when things go wrong — and in production, things always go wrong.

Rust's Result<T, E> is excellent for expected errors: outcomes you anticipated and typed. But real programs also encounter unexpected failures: panics, OOM conditions, and cancelled fibers. id_effect models all of these with a richer type hierarchy.

This chapter introduces Cause (the full error taxonomy), Exit (the terminal outcome of any effect), and the combinators for recovering from both.