zerodds-config

Layered configuration trees with a base file, filename-sorted YAML drop-ins, includes and per-value source locations. This crate loads and merges data. It does not by itself define the DDS runtime's defaults or activation policy.

Maps merge recursively. Scalars and lists replace earlier values, while +name appends to the list at name. Config::origin_of identifies the surviving value's source. The parser accepts a deliberately limited YAML subset, not arbitrary YAML. Include cycles and excessive nesting are errors.

include: accepts a file or directory path, or a block sequence of paths. Included directories use the selected extension filter and filename order. Their fragments may explicitly include further directories. Includes are loaded before the including file's own keys, regardless of the directive's textual position. Paths are relative to the including file and must exist. Missing optional top-level drop-in directories remain valid. A refused Config::push leaves the effective tree and source list unchanged.

Use Loader to select sources, then inspect the resulting Config. The library documentation defines the syntax, precedence and supported include forms. Loader tests exercise these contracts.

Around the files sit two more layers, both in layers.rs: an environment layer beneath them (env_config, from a consumer's table of variable names and paths) and the --set PATH=VALUE flags above them (CommandLine). CommandLine::extract takes --config, --set and --check-config out of a program's arguments; loader_for selects the --config base or the platform's SystemLayout; Loader::load_onto puts the files over the environment; unknown_keys lists paths no consumer reads. The precedence is defaults < environment < files < flags.

Environment interpolation (${VAR}) inside a file is separate from the environment layer: it substitutes text while the file is read.