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BitLogger/docs/api/library-async-logger-bind.md
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2026-06-14 14:00:00 +08:00

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---
name: library-async-logger-bind
group: api
category: facade
update-time: 20260613
description: Attach reusable structured fields to a LibraryAsyncLogger facade through the bind alias while preserving the same underlying async logger state.
key-word:
- async
- library
- bind
- public
---
## Library-async-logger-bind
Attach shared structured fields to a `LibraryAsyncLogger[S]` through the `bind(...)` alias. This is behaviorally identical to `with_context_fields(...)` and exists as a shorter name for common library-facing async context binding.
### Interface
```moonbit
pub fn[S] LibraryAsyncLogger::bind(
self : LibraryAsyncLogger[S],
fields : Array[@bitlogger.Field],
) -> LibraryAsyncLogger[S] {
```
#### input
- `self : LibraryAsyncLogger[S]` - Base facade that should gain shared fields.
- `fields : Array[@bitlogger.Field]` - Structured fields attached to every emitted record.
#### output
- `LibraryAsyncLogger[S]` - New library-facing async wrapper that carries the shared field set.
### Explanation
Detailed rules explaining key parameters and behaviors
- `bind(...)` delegates directly to `with_context_fields(...)`.
- That means the provided `fields` array replaces the previously stored shared field set on the wrapped async logger.
- Shared fields are then prepended to every later log call emitted through the returned facade.
- Sink type, queue state, async config, and failure/lifecycle state remain the same because the method only rewraps the updated async logger value.
- Async state helpers remain hidden behind the narrower facade after rewrapping; use `to_async_logger()` if later code needs them.
- The original facade value is not mutated; a wrapped facade is returned.
- This alias is useful when you prefer shorter chaining syntax in library async code.
### How to Use
Here are some specific examples provided.
#### When Bind Shared Package Context For Async Writes
When a library async logger should carry stable metadata through a flow:
```moonbit
let request_logger = LibraryAsyncLogger::new(@bitlogger.console_sink())
.with_target("sdk")
.bind([@bitlogger.field("request_id", "req-42")])
```
In this example, subsequent async writes automatically include `request_id`.
#### When Prefer Shorter Async Chaining Syntax
When a context-bound child async facade should stay readable:
```moonbit
let worker = LibraryAsyncLogger::new(@bitlogger.console_sink(), target="app")
.child("worker")
.bind([@bitlogger.field("component", "worker")])
```
In this example, `bind(...)` communicates intent without changing underlying behavior.
And because it is only an alias, the resulting facade behaves the same as calling `with_context_fields(...)` directly.
### Error Case
e.g.:
- If `fields` is empty, the returned facade remains valid and simply stores an empty shared field set.
- If duplicate field keys are bound, all copies are preserved for downstream formatting or inspection.
- If callers want event-specific fields without replacing the shared bound set, they should pass those through `log(..., fields=...)` on the returned facade.
### Notes
1. Use `bind(...)` and `with_context_fields(...)` interchangeably; choose the one that reads better in context.
2. This alias exists for ergonomics, not for different semantics.
3. Use `with_context_fields(...)` when the longer name makes the shared-field replacement behavior clearer at the call site.