📝 Document logger composition APIs

docs/api/logger-child.md

@@ -0,0 +1,102 @@
+---
+name: logger-child
+group: api
+category: logging
+update-time: 20260512
+description: Derive a child logger by composing the current target with a child target segment.
+key-word:
+    - logger
+    - target
+    - child
+    - public
+---
+
+## Logger-child
+
+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`.
+
+### Interface
+
+```moonbit
+pub fn[S] Logger::child(self : Logger[S], target : String) -> Logger[S] {}
+```
+
+#### input
+
+- `self : Logger[S]` - Parent logger whose target should be extended.
+- `target : String` - Child target segment or suffix.
+
+#### output
+
+- `Logger[S]` - A new logger whose default target is the composed child path.
+
+---
+
+e.g.:
+```moonbit
+pub fn[S] Logger::child(self : Logger[S], target : String) -> Logger[S] {}
+```
+
+#### input
+
+- `self : Logger[S]` - Parent logger.
+- `target : String` - Child suffix.
+
+#### output
+
+- `Logger[S]` - Logger with combined target.
+
+---
+
+### Explanation
+
+Detailed rules explaining key parameters and behaviors
+
+- If the parent target is empty, the child target becomes the full target.
+- If the child target is empty, the parent target is preserved.
+- If both are non-empty, they are joined with `.`.
+- Sink, min level, and timestamp settings are preserved in the returned logger.
+
+### How to Use
+
+Here are some specific examples provided.
+
+#### When Need Hierarchical Target Naming
+
+When subsystem logs should stay grouped under a shared namespace:
+```moonbit
+let logger = Logger::new(console_sink(), target="service")
+let worker = logger.child("worker")
+```
+
+In this example, `worker` emits records under `service.worker`.
+
+#### When Build Deeply Scoped Loggers Step By Step
+
+When deeper target composition should remain readable:
+```moonbit
+let http = Logger::new(console_sink(), target="app")
+  .child("http")
+  .child("client")
+```
+
+In this example, the final logger target becomes `app.http.client`.
+
+### Error Case
+
+e.g.:
+- If `target` is empty, the returned logger keeps the original parent target.
+
+- If callers need complete replacement rather than composition, `with_target(...)` should be used instead.
+
+### Notes
+
+Notes are here.
+
+1. This is the preferred API for hierarchical logger naming.
+
+2. Composition uses `.` as the separator between parent and child segments.
+
+3. The original logger is not mutated.
+
+4. This API works well with `with_context_fields(...)` to align target and metadata scopes.