Why YAML

YAML is a text format. A decoder can walk a document without a schema. Block collections use indentation. Flow collections use […] and {…} the way JSON does. Comments start with #.

A schema language is optional. This library uses a JSON Schema subset, plus a closed list of YAML extras, to generate Mojo structs. Schema-free work uses YamlValue.

This library implements YAML 1.2 Core. It does not implement YAML 1.1 implicit typing. yes, no, on, off, and NO are strings, not booleans. Python PyYAML is a test oracle only, and it is YAML 1.1 by default, so interop tests use Core-schema-safe values only.

The rest of this page is the subset of the spec that the library implements. Instructions shows how to install and generate code. Examples shows the matching Mojo calls.

Core implicit types

Plain scalars are typed by this table. Quoted scalars stay strings.

Plain text	Core type
null, Null, NULL, ~, empty	null
true / True / TRUE / false / False / FALSE	bool
decimal, 0o…, 0x…	int
decimal with . or exponent, .inf, -.inf, .nan	float
anything else	string

Collections

A block mapping is keys at one indent, each followed by :. A block sequence is lines that start with -. A sequence of mappings is compact: the first key sits on the same line as -, and the later keys line up with that first key.

Flow collections are {key: value} and [a, b].

Anchors

&name marks a node. *name repeats it. Default encode expands aliases. A cycle cannot be expanded and is an error unless keep_anchors is set.

Tags

Core tags !!null, !!bool, !!int, !!float, !!str, !!seq, and !!map are accepted. !!binary is a v1 extra (base64). Every %TAG directive is an error. %YAML 1.2 is ignored.