YAML backend for the cfg protocol.

Load one or more YAML files and receive a Node.IMap[Id.Root] ready for decoding. Parsing is done by snakeyaml-engine with the YAML 1.2 core schema, so ~, null, Null and NULL are nulls, and << merge keys are resolved while composing.

Merging

When multiple URLs are supplied they are overlaid in order — later files take precedence over earlier ones (last-wins). Mappings are merged recursively, key by key; scalars and sequences are replaced outright rather than concatenated. An empty argument list yields an empty root map. Unlike the HOCON backend there is no substitution step: a YAML file is taken exactly as written.

Tags

Unlike the HOCON backend, this one reports tags. tag holds the tag as it was written in the source, already expanded through the tag handles in effect (!!int becomes tag:yaml.org,2002:int, a local !postgres stays !postgres). It is None whenever the tag was inferred by the resolver rather than written down, and also for the non-specific tag !, which says nothing about the type. Scalars are always carried as raw strings — a tag never converts a value, it only records what the author asked for, and acting on it is the decoder's business.

Duplicate keys

A mapping that repeats a key keeps the last occurrence, matching the last-wins rule used for merging.

Thread safety

apply builds its own parser per call and shares no mutable state; the returned tree is immutable.