Crate ryml

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ryml: Rapid YAML Bindings for Rust

GPL 3 version GitHub issues

Rapid YAML/ryml is a C++ library to parse and emit YAML, and do it fast. The ryml crate provides fairly thin but safe (I think) FFI bindings to bring this power to Rust.

This crate is currently is early, early alpha. Not all of the C++ API is covered, and some of it probably will not be, but the core functionality is all in place (indeed, much more than the official Python bindings).

A basic example of how to use this crate:

use ryml::Tree;

static SRC: &str = r#"
Hello: World
Names: [Caleb, Voronwe, 'Nicene Nerd']
Credo: !!apostolicus
  - in Deum, Patrem omnipotentem
  - et in Iesum Christum, Filium eius unicum
  - in Spiritum Sanctum

let mut tree = Tree::parse(SRC)?;
assert_eq!(10, tree.len());

// First, the low-level, index-based API
let root_id = tree.root_id()?;
assert_eq!(root_id, 0);
assert_eq!(tree.num_children(root_id)?, 3);
for i in 0..tree.num_children(root_id)? {
   let child_id = tree.child_at(root_id, i)?;
   println!("{}", tree.key_scalar(child_id)?.scalar); // "Hello", "Names", "Credo"

// Next, the high-level, NodeRef API
let world = tree.root_ref()?.get("Hello")?;
assert_eq!(world.val()?, "World");
let credo = tree.root_ref()?.get("Credo")?;
assert_eq!(credo.val_tag()?, "!!apostolicus");    
for node in credo.iter()? {
   println!("{}", node.val()?);

// Mutate the tree
    let mut root_ref_mut = tree.root_ref_mut()?;
    let mut credo = root_ref_mut.get_mut("Credo")?;
    let mut amen = credo.get_mut(3)?; // A new index

// Serialize the tree back to a string
let new_text = tree.emit()?;

static END: &str = r#"Hello: World
  - Caleb
  - Voronwe
  - 'Nicene Nerd'
Credo: !!apostolicus
  - 'in Deum, Patrem omnipotentem'
  - 'et in Iesum Christum, Filium eius unicum'
  - in Spiritum Sanctum
  - Amen

assert_eq!(new_text, END);


For more usage information, see the full documentation.


  • An entry in the Tree representing all the data for a YAML node.
  • A reference to a node in the tree.
  • A view of scalar data for a node, containing the tag, anchor, and scalar value.
  • A bitmask for marking node types.
  • Represents a parsed YAML tree


  • Error type for this crate


  • Represents the pseudo-index of a node that does not exist.