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Nanaloveyuki
75 lines
2.2 KiB
Markdown
75 lines
2.2 KiB
Markdown
---
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name: record-patch
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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 patch function type used for record transformation before sink write.
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key-word:
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- patch
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- record
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- transform
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- public
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---
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## Record-patch
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`RecordPatch` is the public function type used for record transformation. It represents any function that receives a `Record` and returns a new `Record`, and it is the shared shape behind helpers such as `set_target(...)`, `prefix_message(...)`, and `append_fields(...)`.
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### Interface
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```moonbit
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pub type RecordPatch = @utils.RecordPatch
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```
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#### output
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- `RecordPatch` - A function type equivalent to `(Record) -> Record`.
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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) -> Record` can be used wherever a `RecordPatch` is expected.
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- Patches are used by `Logger::with_patch(...)`, `patch_sink(...)`, and helper constructors in `patchers.mbt`.
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- The alias exists to make record-rewrite 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 Record Rewrite Step
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When a logger should rewrite each record before sink write:
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```moonbit
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let patch : RecordPatch = fn(rec) { rec.with_message("[custom] " + rec.message) }
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let logger = Logger::new(console_sink()).with_patch(patch)
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```
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In this example, the alias makes the custom transformation shape explicit.
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#### When Compose Existing Patch Helpers
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When several reusable transformations should be combined:
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```moonbit
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let patch : RecordPatch = compose_patches([
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set_target("service.worker"),
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prefix_message("[worker] "),
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])
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```
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In this example, the alias describes the common type returned by the patch helper constructors.
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### Error Case
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e.g.:
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- A patch that drops important fields or overwrites the target can change downstream observability unexpectedly.
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- A patch that allocates heavily or copies too much data can add cost to every write path.
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### Notes
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1. Use the alias when function signatures should communicate transformation intent clearly.
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2. Helper constructors in `patchers.mbt` are usually preferable to handwritten patches for common cases.
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