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Nanaloveyuki
87 lines
2.3 KiB
Markdown
87 lines
2.3 KiB
Markdown
---
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name: target-has-prefix
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group: api
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category: filtering
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update-time: 20260512
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description: Create a reusable record predicate that matches records by target prefix.
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key-word:
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- target
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- filter
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- predicate
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- public
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---
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## Target-has-prefix
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Create a `RecordPredicate` that returns `true` when a record target starts with a given prefix. This helper is commonly used with `with_filter(...)`, `filter_sink(...)`, `all_of(...)`, and routing logic.
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### Interface
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```moonbit
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pub fn target_has_prefix(prefix : String) -> RecordPredicate {}
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```
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#### input
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- `prefix : String` - Prefix expected at the start of `rec.target`.
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#### output
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- `RecordPredicate` - Predicate that matches records whose target starts with the given prefix.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- Matching is based on `String::has_prefix(...)` over the full target string.
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- This helper does not modify records; it only returns a reusable predicate.
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- Prefix-based matching is especially useful when using hierarchical targets such as `app.worker.job`.
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- It composes cleanly with `all_of(...)`, `any_of(...)`, and `not_(...)`.
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### How to Use
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Here are some specific examples provided.
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#### When Filter A Target Namespace
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When a logger should keep one target subtree:
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```moonbit
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let logger = Logger::new(console_sink(), target="app")
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.with_filter(target_has_prefix("app.worker"))
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```
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In this example, only records under `app.worker...` remain visible.
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And child targets continue to compose naturally.
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#### When Combine With Level Rules
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When both namespace and severity matter:
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```moonbit
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let predicate = all_of([
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target_has_prefix("service.api"),
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level_at_least(Level::Warn),
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])
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```
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In this example, the predicate stays reusable across multiple loggers or sinks.
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### Error Case
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e.g.:
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- If `prefix` is empty, the predicate effectively matches every target because every string starts with an empty prefix.
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- If no target matches the prefix, the predicate simply returns `false` for those records.
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### Notes
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Notes are here.
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1. This helper is most useful when targets are structured hierarchically.
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2. Prefer this helper over ad hoc inline prefix logic for readability.
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3. Use `target_is(...)` instead when exact equality is required.
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4. Combine with `message_contains(...)` or field predicates for narrower routing rules.
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