---
name: runtime-sink-file-path
group: api
category: runtime
update-time: 20260707
description: Read the effective file path used by a file-backed RuntimeSink, with a compatibility fallback for non-file sinks.
key-word:
    - runtime
    - sink
    - file
    - public
---

## Runtime-sink-file-path

Read the effective file path used by a file-backed `RuntimeSink`. This helper is useful for diagnostics, support output, and confirming which direct runtime file sink is active.

`file_path()` is the compatibility form. For truthful file-semantics detection, prefer `file_path_or_none()`.

### Interface

```moonbit
pub fn RuntimeSink::file_path(self : RuntimeSink) -> String {
```

#### input

- `self : RuntimeSink` - Runtime sink whose file path should be inspected.

#### output

- `String` - Effective runtime file path, or an empty string for non-file sinks.

### Explanation

Detailed rules explaining key parameters and behaviors

- Plain `File` runtime variants return their current file path.
- `QueuedFile` runtime variants forward the wrapped inner file sink path.
- Non-file runtime variants return an empty string.
- This fallback keeps older callers source-compatible, but it does not distinguish “not file-backed” from “file-backed with an empty-looking value”.
- New diagnostic code should prefer `file_path_or_none()` when it must avoid fallback values.
- This helper is observation-only and does not modify file state.

### How to Use

Here are some specific examples provided.

#### When Need Direct Runtime Path Diagnostics

When code owns a `RuntimeSink` and should show which file is active:
```moonbit
println(sink.file_path())
```

In this example, operators can verify the effective destination path directly from the runtime sink.

#### When Build Support Output From A Runtime Sink

When application state dumps should include file destination info:
```moonbit
let path = sink.file_path()
```

In this example, the path can be surfaced without reading broader runtime state.

### Error Case

e.g.:
- If the runtime sink is not file-backed, the method returns an empty string.

- If callers need a truthful file-semantics check, use `file_path_or_none()`.

- If callers need richer file status than just the path, `file_state_or_none()` or `file_runtime_state()` is the better API.

### Notes

1. Use this helper for direct runtime path visibility on `RuntimeSink` values.

2. Empty string is a compatibility fallback; `file_path_or_none()` is the recommended truthful API.