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BitLogger/docs/api/logger-log.md
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2026-05-12 16:06:00 +08:00

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---
name: logger-log
group: api
category: logging
update-time: 20260512
description: Emit a record through the logger with an explicit level, message, optional fields, and optional target override.
key-word:
- logger
- log
- sync
- public
---
## Logger-log
Emit a record through the synchronous logger with an explicit level and message. This is the core write API behind all level-specific convenience methods.
### Interface
```moonbit
pub fn[S : Sink] Logger::log(
self : Logger[S],
level : Level,
message : String,
fields~ : Array[Field] = [],
target? : String = "",
) -> Unit {}
```
#### input
- `self : Logger[S]` - Logger that should emit the record.
- `level : Level` - Severity level for the record.
- `message : String` - Log message text.
- `fields : Array[Field]` - Optional structured fields attached to the record.
- `target : String` - Optional per-call target override.
#### output
- `Unit` - No return value. The record is either skipped by level gating or written to the sink pipeline.
### Explanation
Detailed rules explaining key parameters and behaviors
- The logger checks `is_enabled(level)` before building and writing the record.
- If `target` is empty, the logger's stored default target is used.
- If timestamping is enabled, `@env.now()` is captured and stored in the emitted record.
- Context, filter, patch, and queue wrappers act through the sink pipeline after the record is constructed.
### How to Use
Here are some specific examples provided.
#### When Need Full Per-call Control
When code should choose level, fields, and target explicitly:
```moonbit
logger.log(
Level::Info,
"worker started",
fields=[field("job", "sync")],
target="service.worker",
)
```
In this example, the call site controls every major record property.
#### When Build Higher-level Wrappers
When application code defines its own logging helpers:
```moonbit
fn log_retry(logger : Logger[ConsoleSink], attempt : Int) -> Unit {
logger.log(Level::Warn, "retry scheduled", fields=[field("attempt", attempt.to_string())])
}
```
In this example, `log(...)` acts as the shared primitive under custom wrappers.
### Error Case
e.g.:
- If `level` is below the current minimum threshold, the method returns without writing a record.
- If `target` is empty and the logger default target is also empty, the emitted record simply has no target label.
### Notes
1. Use this API when the call site needs full control instead of a fixed severity helper.
2. This method is synchronous; any buffering behavior only comes from the wrapped sink type.