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Nanaloveyuki
75 lines
2.2 KiB
Markdown
75 lines
2.2 KiB
Markdown
---
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name: record-predicate
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group: api
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category: helper
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update-time: 20260613
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description: Public predicate function type used for logger filtering and record selection.
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key-word:
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- predicate
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- filter
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- record
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- public
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---
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## Record-predicate
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`RecordPredicate` is the public function type used for record filtering. It represents any function that receives a `Record` and returns `Bool`, and it is the shared shape behind helpers such as `level_at_least(...)`, `target_is(...)`, and `filter_sink(...)`.
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### Interface
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```moonbit
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pub type RecordPredicate = @utils.RecordPredicate
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```
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#### output
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- `RecordPredicate` - A function type equivalent to `(Record) -> Bool`.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- This is a type alias, not a new runtime wrapper.
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- Any function matching `(Record) -> Bool` can be used wherever a `RecordPredicate` is expected.
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- Predicates are used by `Logger::with_filter(...)`, `filter_sink(...)`, and helper constructors in `filters.mbt`.
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- The alias exists to make filtering APIs clearer and easier to read in public signatures.
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### How to Use
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Here are some specific examples provided.
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#### When Need A Custom Logger Filter
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When a logger should only accept records that match local rules:
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```moonbit
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let predicate : RecordPredicate = fn(rec) { rec.level.priority() >= Level::Warn.priority() }
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let logger = Logger::new(console_sink()).with_filter(predicate)
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```
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In this example, the alias makes the custom filter shape explicit.
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#### When Compose Existing Predicate Helpers
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When several reusable filters should be combined:
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```moonbit
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let predicate : RecordPredicate = all_of([
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target_has_prefix("service"),
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not_(message_contains("ignore")),
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])
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```
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In this example, the alias describes the common type returned by the helper constructors.
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### Error Case
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e.g.:
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- A predicate that always returns `false` is valid but will suppress all matching writes.
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- A predicate that is expensive to evaluate can add avoidable cost to every filtered record.
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### Notes
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1. Use the alias when function signatures should communicate filtering intent clearly.
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2. Helper constructors in `filters.mbt` are usually preferable to handwritten predicates for common cases.
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