---
title: "Read YAML without a schema"
url: https://recursiveai.org/oneyaml/rust/cookbook/read-yaml-without-a-schema/
description: "Read YAML of unknown shape into oneyaml::Value in Rust, index it by key and position, and walk a mapping in file order."
version: "1.2.0"
---

# Read YAML without a schema

When you don’t have a struct for the data, read it into `oneyaml::Value` and index it like `serde_json::Value`.

```rust
use oneyaml::Value;

fn main() -> Result<(), oneyaml::Error> {
    let src = "\
server:
  host: 0.0.0.0
  port: 8080
features: [auth, metrics]
limits:
  cpu: 0.5
  memory: 512Mi
";
    let v: Value = oneyaml::from_str(src)?;

    // Index by key or position; a missing key gives Value::Null, not a panic.
    println!("host     = {:?}", v["server"]["host"].as_str());
    println!("port     = {:?}", v["server"]["port"].as_u64());
    println!("feature0 = {:?}", v["features"][0].as_str());
    println!("missing  = {:?}", v["server"]["tls"].as_str());

    // Walk a mapping in file order.
    if let Some(limits) = v["limits"].as_mapping() {
        for (k, val) in limits {
            println!("limit {} = {:?}", k.as_str().unwrap_or("?"), val);
        }
    }
    Ok(())
}
```

```text
host     = Some("0.0.0.0")
port     = Some(8080)
feature0 = Some("auth")
missing  = None
limit cpu = Number(0.5)
limit memory = String("512Mi")
```

## Good to know

- Indexing with `v["key"]` or `v[0]` never panics: a missing key or index gives `Value::Null`, so `as_str()` and friends return `None`.
- A mapping keeps the order of the file.
- `0.5` is a number and `512Mi` a string: the Core schema decides, the same way for `Value` as for a struct.
- To read values and also edit the file, use a `Document` instead: `doc.get_path("server.port")` reads a node with its span, comments and style ([edit YAML and keep its comments](https://recursiveai.org/oneyaml/rust/cookbook/edit-yaml-keep-comments/)).
- `oneyaml::from_value` turns a `Value` into a typed struct later, once you know which one it is.