CODEWITHSUNDEEP
yaml
gavenkoa
gavenkoa

Reputation: 48743

YAML merge multiply anchors into the same map

It is not clear if it is possible to use YAML merge key operator several time for the same map.

The confusion comes from:

Original spec says:

The “<<” merge key is used to indicate that all the keys of one or more specified maps should be inserted into the current map.

but they don't provide an example and I cannot find it elsewhere. Like:

defs:
- map1: &map1-ref
    key1: val1
- map2: &map2-ref
    key2: val2

config:
  database:
    <<: *map1-ref
    <<: *map2-ref
    key3: val3

Which major parsers support multiply merges (python/ruby/java/node)?

UPDATE Python 3 check code:

import yaml
with open("my.yml") as f:
    y = yaml.safe_load(f)
    print(y)

UPDATE 2 @flyx's suggested syntax can be written in an alternative way:

config:
  database:
    <<:
      - *map1-ref
      - *map2-ref
    key3: val3

Upvotes: 9

Views: 15910

Answers (1)

flyx
flyx

Reputation: 39638

While a quick test shows that PyYAML does support this, I would advise against it since having multiple identical keys (<<) in the same mapping violates the core YAML spec (both 1.1 and 1.2).

Moreover, it is unnecessary since the merge key can take multiple maps as arguments:

defs:
- map1: &map1-ref
    key1: val1
- map2: &map2-ref
    key2: val2

config:
  database:
    <<: [*map1-ref, *map2-ref]
    key3: val3

If you use this syntax, the question boils down to „which implementations support the merge key“. I can't give a full list but here's what I know:

  • Supports it:
    • PyYAML
    • Java's SnakeYAML
    • Ruby's Psych
    • Perl's YAML::PP
    • go-yaml
  • Does not support it:
    • yaml-cpp (GH issue)
    • libyaml (because its main interface is the event stream which is too low-level)
    • NimYAML (my work)
    • AdaYaml (my work)

Upvotes: 20

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