Testing Patterns & Effect Abstraction

Short version: model your “effects” as traits, inject them, keep core logic pure, and provide real + fake implementations. That’s the idiomatic Rust way; free monads aren’t a thing here.

Pattern

• Define algebras as traits (ports).
• Implement adapters for prod (HTTP, DB, clock, FS) and for tests (fakes/mocks).
• Inject via generics (zero-cost, monomorphized) or trait objects (dyn Trait) when you need late binding.
• Keep domain functions pure; pass in effect results or tiny capability traits.

Minimal sync example

use std::time::{SystemTime, UNIX_EPOCH};

pub trait Clock {