YAML

YAML 1.2 is read and written by the yaml package, which has the same shape as the json and toml modules: a YamlValue to look values up in and change, clang-style errors at the line and column, documents that keep their comments when edited, and struct conversion with the serialize attributes. It is a package rather than part of the runtime because few programs need YAML; add it with pepper package add justinboswell/pepper-yaml --name yaml.

import yaml;

main(): int {
    var parsed = yaml.parse("server:\n  port: 8080\n  debug: no\n", path = "config.yaml");
    if (parsed.is_err()) {
        print(parsed.error().to_string());
        return 1;
    }
    var root = parsed.value();
    print(root["server"]["port"].as_int().value());       # 8080
    print(root["server"]["debug"].as_string().value());   # no
    root["server"]["port"] = yaml.YamlValue.Integer(9090);
    print(root.to_string());
    return 0;
}

• The core schema: yes, no, on and off are strings, not booleans, so NO (Norway) stays NO.
• Safe for untrusted text: only the core schema’s tags (!!str, !!int, …) are read, and any other tag is an error; aliases may not stand for more than max_alias_nodes nodes, which stops the “billion laughs” attack.
• Merge keys: yaml.parse(text, merge_keys = true) applies YAML 1.1’s << (<<: *defaults), which CI and compose files use to reuse a mapping; the mapping’s own keys win, and merged nodes count toward max_alias_nodes. By default << is an ordinary key, as YAML 1.2 says.
• Several documents in one stream (---) are read by yaml.parse_all and written by yaml.write_all.
• yaml.parse_document keeps comments, key order, flow or block style and scalar styles through edits, as json.parse_document keeps a JSONC file’s comments.
• yaml.encode and yaml.decode<T> convert structs exactly as json.encode and json.decode do.

The package’s README lists what it reads and its limits: a mapping’s keys are strings, and there is no streaming reader yet.