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Nanaloveyuki
80 lines
2.3 KiB
Markdown
80 lines
2.3 KiB
Markdown
---
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name: runtime-sink-file-default-policy-or-none
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group: api
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category: runtime
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update-time: 20260707
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description: Read the default runtime file policy from a RuntimeSink only when it is actually file-backed.
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key-word:
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- runtime
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- sink
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- file
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- truthful
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---
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## Runtime-sink-file-default-policy-or-none
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Read the default runtime file policy from a `RuntimeSink` only when it is actually file-backed.
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This is the truthful companion to `file_default_policy()`. Prefer it when default-policy inspection should not fabricate a fallback object on non-file sinks.
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### Interface
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```moonbit
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pub fn RuntimeSink::file_default_policy_or_none(self : RuntimeSink) -> FileSinkPolicy? {
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```
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#### input
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- `self : RuntimeSink` - Runtime sink whose default file policy should be inspected.
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#### output
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- `FileSinkPolicy?` - `Some(policy)` when the runtime sink is file-backed, otherwise `None`.
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### Explanation
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Detailed rules explaining key parameters and behaviors
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- Plain `File` runtime variants return `Some(default_policy)`.
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- `QueuedFile` runtime variants forward the wrapped inner file sink default policy as `Some(default_policy)`.
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- Non-file runtime variants return `None`.
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- This helper is useful when callers need to compare runtime drift or restore defaults without fabricating file semantics.
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### How to Use
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Here are some specific examples provided.
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#### When Need Truthful Baseline Policy Visibility
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When tooling should inspect defaults only for real file-backed sinks:
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```moonbit
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let defaults = sink.file_default_policy_or_none()
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```
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In this example, `None` means the runtime sink has no file default policy.
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#### When Compare Current And Default Policy Truthfully
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When diagnostics should avoid fallback policy objects:
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```moonbit
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match (sink.file_policy_or_none(), sink.file_default_policy_or_none()) {
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(Some(current), Some(defaults)) => ignore((current, defaults))
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_ => ()
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}
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```
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In this example, comparisons happen only when real file semantics exist.
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### Error Case
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e.g.:
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- If the runtime sink is not file-backed, the method returns `None`.
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- If callers only need a drift boolean, `file_policy_matches_default()` remains simpler.
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### Notes
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1. Prefer this helper over `file_default_policy()` for truthful runtime diagnostics.
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2. `file_default_policy()` remains available as the compatibility fallback API.
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