---
name: logger-to-library-logger
group: api
category: facade
update-time: 20260613
description: Convert a full sync logger into the narrower library-facing sync facade.
key-word:
    - logger
    - library
    - facade
    - public
---

## Logger-to-library-logger

Convert `Logger[S]` into `LibraryLogger[S]`. This keeps the same sink and logging behavior while projecting the value onto the smaller library-facing sync surface.

### Interface

```moonbit
pub fn[S] Logger::to_library_logger(self : Logger[S]) -> LibraryLogger[S] {}
```

#### input

- `self : Logger[S]` - Full sync logger to project into the library facade.

#### output

- `LibraryLogger[S]` - Narrower library-facing wrapper over the same logger state.

### Explanation

Detailed rules explaining key parameters and behaviors

- This conversion does not rebuild the sink or change the logger configuration.
- Target, min level, timestamp behavior, and sink wiring are preserved.
- The returned facade keeps library-oriented write APIs such as `info(...)`, `warn(...)`, and `error(...)`.

### How to Use

Here are some specific examples provided.

#### When Need To Return A Narrower Library Type

When internal setup uses the full logger API but the exposed result should stay smaller:
```moonbit
let logger = Logger::new(console_sink(), target="lib")
let public_logger = logger.to_library_logger()
```

In this example, `public_logger` keeps the same logging behavior but exposes the library facade.

### Error Case

e.g.:
- If callers later need APIs outside the library facade, they must unwrap with `to_logger()`.

- The conversion does not remove existing target or field bindings from the original logger.

### Notes

1. Use this when library boundaries should avoid exposing the full sync logger surface.

2. This is a projection API, not a copy or rebuild step.