Inconsistent Key Resolution in YAML 1.2 Merge Key (

Question

The YAML 1.2 specification allows the merge key << to inherit key-value pairs from one or more source mappings. While the specification ensures that keys explicitly defined in the target mapping override any merged values, it does not provide a definitive rule for precedence when multiple merge sources contain the same key.

This ambiguity creates uncertainty when utilizing a list of merge sources. Different parser implementations, such as SnakeYAML and js-yaml, may resolve duplicate keys across multiple sources using different precedence logic (e.g., first-source vs. last-source precedence).

Additionally, the behavior of nested merge resolution—where a source mapping itself contains a merge key—is not uniformly standardized across libraries.

Which source takes precedence when multiple mappings in a merge list contain the same key?
  Is there a standardized order for resolving nested merge keys before they are applied to the target mapping?

Tasadduq Burney · Answer

Answer
The YAML 1.2 specification does not define an order for resolving duplicate keys that appear in multiple merge sources; therefore, precedence is implementation‑dependent.
Confirmed facts

The spec guarantees that keys explicitly present in the target mapping override any values merged from << sources.
  It leaves the precedence among equal keys in the merge sequence unspecified.
  For nested merge keys (a source that itself contains <<), the spec expects the inner merge to be resolved recursively before the source’s keys are merged, but this recursion is not uniformly implemented.

Likely explanation
Many popular parsers (e.g., SnakeYAML, js‑yaml) adopt a “last‑source‑wins” rule: later items in the merge list override earlier ones. Older or alternative parsers may use “first‑source‑wins”. Because the spec is silent, both behaviors are conformant.
Steps to obtain predictable results

If you control the YAML, avoid defining the same key in more than one merge source.
  If duplicates are unavoidable, order the merge list so that the source you want to win appears last (assuming your parser follows last‑source‑wins).
  Test the document with your actual parser to verify the outcome.

Missing diagnostic detail
To give a definitive recommendation, please specify which YAML library (and version) you are using, as precedence differs between implementations.

Inconsistent Key Resolution in YAML 1.2 Merge Key (<<) Implementations

1 answer

Answer

Confirmed facts

Likely explanation

Steps to obtain predictable results

Missing diagnostic detail

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