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Nanaloveyuki
91 lines
3.4 KiB
Markdown
91 lines
3.4 KiB
Markdown
---
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name: library-logger-with-context-fields
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group: api
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category: facade
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update-time: 20260613
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description: Attach reusable structured fields to a LibraryLogger facade while extending the visible sink type to ContextSink.
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key-word:
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- library
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- facade
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- fields
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- public
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---
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## Library-logger-with-context-fields
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Bind shared structured fields to a `LibraryLogger[S]`. This is the library-facing API for attaching stable metadata such as component name, plugin id, or request scope without repeating those fields on every log call.
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### Interface
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```moonbit
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pub fn[S] LibraryLogger::with_context_fields(
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self : LibraryLogger[S],
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fields : Array[Field],
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) -> LibraryLogger[ContextSink[S]] {
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```
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#### input
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- `self : LibraryLogger[S]` - Base facade that should gain shared fields.
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- `fields : Array[Field]` - Structured fields stored in the added `ContextSink` and prepended to each emitted record.
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#### output
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- `LibraryLogger[ContextSink[S]]` - New library-facing facade wrapping the original sink with context-field merging behavior.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- This API delegates to the wrapped logger's `with_context_fields(...)` behavior and then narrows the result back to `LibraryLogger`.
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- Context fields are merged at write time rather than by mutating previously created records.
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- The returned facade carries `ContextSink[S]` because the sink pipeline is extended around the previous sink instead of keeping the original visible sink type `S`.
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- Minimum level, target, and timestamp behavior are preserved while the sink pipeline changes from `S` to `ContextSink[S]`.
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- Broader composition helpers remain hidden behind the narrower facade after rewrapping; use `to_logger()` if later code needs them.
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- `bind(...)` is an ergonomic alias for this same behavior.
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### How to Use
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Here are some specific examples provided.
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#### When Need Stable Library Metadata On Every Record
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When a package should attach fixed metadata to all emitted records:
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```moonbit
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let logger = default_library_logger()
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.with_target("plugin")
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.with_context_fields([field("plugin", "alpha"), field("region", "cn")])
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```
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In this example, both fields are automatically included on every later write.
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#### When Combine Target Scoping And Shared Fields
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When a subsystem facade should carry both a target and stable context:
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```moonbit
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let worker = LibraryLogger::new(console_sink(), target="sdk")
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.child("worker")
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.with_context_fields([field("component", "worker")])
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```
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In this example, target composition and field binding stay separate but compose cleanly.
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And the returned facade keeps the same logger configuration while exposing the widened sink type `ContextSink[S]`.
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### Error Case
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e.g.:
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- If `fields` is empty, the returned facade remains valid and simply stores an empty shared field set inside `ContextSink[S]`.
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- If duplicate field keys are provided, all fields are still emitted for downstream formatting or inspection.
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- If callers want event-specific fields without changing the shared bound set, they should pass those through `log(..., fields=...)` on the returned facade.
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### Notes
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1. Use this for stable shared metadata, not highly dynamic event-specific values.
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2. The narrower `LibraryLogger` surface is preserved after context binding, even though the visible sink type changes to `ContextSink[S]`.
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3. Use `bind(...)` when the shorter alias reads better in chained library code.
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