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Nanaloveyuki
103 lines
2.4 KiB
Markdown
103 lines
2.4 KiB
Markdown
---
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name: logger-child
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group: api
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category: logging
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update-time: 20260512
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description: Derive a child logger by composing the current target with a child target segment.
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key-word:
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- logger
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- target
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- child
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- public
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---
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## Logger-child
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Create a child logger by composing the current target with another target segment. This is the standard API for hierarchical target naming such as `app.worker` or `service.http.client`.
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### Interface
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```moonbit
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pub fn[S] Logger::child(self : Logger[S], target : String) -> Logger[S] {}
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```
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#### input
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- `self : Logger[S]` - Parent logger whose target should be extended.
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- `target : String` - Child target segment or suffix.
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#### output
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- `Logger[S]` - A new logger whose default target is the composed child path.
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---
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e.g.:
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```moonbit
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pub fn[S] Logger::child(self : Logger[S], target : String) -> Logger[S] {}
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```
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#### input
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- `self : Logger[S]` - Parent logger.
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- `target : String` - Child suffix.
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#### output
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- `Logger[S]` - Logger with combined target.
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---
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- If the parent target is empty, the child target becomes the full target.
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- If the child target is empty, the parent target is preserved.
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- If both are non-empty, they are joined with `.`.
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- Sink, min level, and timestamp settings are preserved in the returned logger.
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### How to Use
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Here are some specific examples provided.
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#### When Need Hierarchical Target Naming
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When subsystem logs should stay grouped under a shared namespace:
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```moonbit
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let logger = Logger::new(console_sink(), target="service")
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let worker = logger.child("worker")
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```
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In this example, `worker` emits records under `service.worker`.
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#### When Build Deeply Scoped Loggers Step By Step
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When deeper target composition should remain readable:
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```moonbit
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let http = Logger::new(console_sink(), target="app")
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.child("http")
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.child("client")
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```
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In this example, the final logger target becomes `app.http.client`.
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### Error Case
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e.g.:
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- If `target` is empty, the returned logger keeps the original parent target.
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- If callers need complete replacement rather than composition, `with_target(...)` should be used instead.
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### Notes
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Notes are here.
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1. This is the preferred API for hierarchical logger naming.
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2. Composition uses `.` as the separator between parent and child segments.
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3. The original logger is not mutated.
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4. This API works well with `with_context_fields(...)` to align target and metadata scopes.
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