# Logging (`id_effect_logger`)
Workspace crate **`id_effect_logger`** provides an injectable **`EffectLogger`** service: log lines are **effect steps** that read the logger capability from **`R`**, so formatting and sinks stay composable and testable.
## Usage shape
- `~EffectLogger` inside **`effect!`** (with `|r|`) to obtain the handle, then **`info`**, **`warn`**, … on static or dynamic messages (`impl Into<Cow<'static, str>>`).
- **`provide_effect_logger`** (and related constructors) build a [`ProviderSpec`](../../src/capability/provider.rs) you pass to **`run_with`** / **`build_env`**.
```rust
use id_effect::{Effect, effect, caps, provide, run_with};
fn app() -> Effect<(), (), caps!(EffectLogger)> {
effect!(|r| {
let log = *~EffectLogger;
~ log.info("hello");
()
})
}
run_with([provide!(TracingLoggerLive)], app())?;
```
## Backends
The crate ships pipeline pieces such as **structured JSON**, **tracing** integration, and **composite** backends—see `cargo doc -p id_effect_logger` for **`LogBackend`**, **`JsonLogBackend`**, **`TracingLogBackend`**, **`CompositeLogBackend`**.
## Relation to the rest of Part II
Logging is just another **capability**: same mental model as [Capability services](./ch05-00-tags-context.md) and [Providing Services](./ch07-03-providing-services.md). Swap backends in tests via `provide!(…)` instead of silencing `println!`.
## Further reading
- `cargo doc --open -p id_effect_logger`