add << merge key to "spread" mappings into a mapping.
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In order to gracefully compose data structures, you need a way to insert an entire set of mappings into another at a single place. This is equivalent to the javascript `...` operator. YAML allows support for this with the language independent type "merge-key" feature. This implements the spec here https://yaml.org/type/merge.html Thus evaluating this example: ```yaml composed: <<: one: 1 two: 2 <<: three: 3 four: 4 <<: - five: 5 - six: 6 ``` is: ```yaml composed: one: 1 two: 2 three: 3 four: 4 five: 5 six: 6 ``` Although ubiquitous, this syntax is officially deprecated and will be replaced by something else in YAML 1.3 but nobody is for sure what that will be.
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Motivation
In order to gracefully compose data structures, you need a way to insert an entire set of mappings into another at a single place. This is equivalent to the javascript
...operator.YAML allows support for this with the language independent type "merge-key" feature.
Approach
This implements the spec here https://yaml.org/type/merge.html
Thus evaluating this example:
is:
Learning
Although ubiquitous, this syntax is officially deprecated and will be replaced by something else in YAML 1.3 but nobody is for sure what that will be.