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BitLogger/docs/api/library-logger-warn.md
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2026-06-13 20:42:12 +08:00

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---
name: library-logger-warn
group: api
category: facade
update-time: 20260613
description: Emit a warn-level record through a LibraryLogger facade using the warning severity shortcut.
key-word:
- library
- facade
- warn
- public
---
## Library-logger-warn
Emit a warn-level record through the library-facing sync logger. This is the convenience wrapper for `log(Level::Warn, ...)` on `LibraryLogger[S]`.
### Interface
```moonbit
pub fn[S : Sink] LibraryLogger::warn(
self : LibraryLogger[S],
message : String,
fields~ : Array[Field] = [],
) -> Unit {
```
#### input
- `self : LibraryLogger[S]` - Library-facing logger that should emit the warning record.
- `message : String` - Warning message text.
- `fields : Array[Field]` - Optional structured fields attached to the record.
#### output
- `Unit` - No return value. The record is handled according to the current threshold and wrapped sink pipeline.
### Explanation
Detailed rules explaining key parameters and behaviors
- This helper delegates to `log(Level::Warn, ...)` on the wrapped logger.
- Warning records are useful for abnormal but non-fatal conditions.
- Per-call target override is not exposed here; use `log(...)` when that is required.
- All logger wrappers still participate normally in the write path.
### How to Use
Here are some specific examples provided.
#### When Signal A Recoverable Library Problem
When an operation degraded but still continued:
```moonbit
logger.warn("cache miss ratio increased")
```
In this example, the event is elevated above normal information without being treated as a hard failure.
#### When Attach Structured Warning Context
When a warning should include machine-readable detail:
```moonbit
logger.warn("retry scheduled", fields=[field("attempt", "3")])
```
In this example, the warning remains easy to filter and inspect later.
### Error Case
e.g.:
- If the logger minimum level is above `Warn`, the call returns without writing a record.
- If a target override is needed at this call site, use `log(...)` instead of this shortcut.
### Notes
1. Use this helper for degraded or suspicious states that do not stop execution.
2. Warning logs are often a practical signal threshold for alerting or separate routing.