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Nanaloveyuki
78 lines
2.0 KiB
Markdown
78 lines
2.0 KiB
Markdown
---
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name: field
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group: api
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category: record
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update-time: 20260512
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description: Create a single structured field value from a key and value pair.
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key-word:
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- field
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- record
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- helper
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- public
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---
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## Field
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Create a single `Field` value from a key and value pair. This is the basic helper used throughout logger, patch, and filter APIs whenever structured metadata needs to be attached to a record.
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### Interface
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```moonbit
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pub fn field(key : String, value : String) -> Field {}
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```
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#### input
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- `key : String` - Field key name.
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- `value : String` - Field value text.
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#### output
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- `Field` - Structured field value ready to be used in a record or field array.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- The helper constructs a `Field` directly from the provided key and value.
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- No deduplication, normalization, or validation is applied.
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- This is the standard primitive used by `fields(...)`, `append_fields(...)`, and all log write APIs that accept structured fields.
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- The created field preserves the original input strings exactly.
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### How to Use
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Here are some specific examples provided.
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#### When Add One Inline Event Field
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When a single structured field is enough:
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```moonbit
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logger.info("accepted", fields=[field("user", "alice")])
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```
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In this example, the event carries one explicit structured attribute.
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#### When Build Shared Context Fields
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When binding one reusable metadata field to a logger:
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```moonbit
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let logger = Logger::new(console_sink(), target="audit")
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.bind([field("service", "billing")])
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```
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In this example, the field becomes part of every later emitted record.
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### Error Case
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e.g.:
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- If `key` is empty, the resulting field still exists and keeps the empty string key.
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- If `value` is empty, the field remains valid and simply stores an empty string value.
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### Notes
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1. Use this helper when only one field is needed and tuple syntax would be less direct.
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2. Prefer stable field keys so filters and downstream tooling remain predictable.
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