Hi guys, Quick q: are yaml merge keys supported? ...

# hoplite

[David Silva]

Hi guys,

Quick q: are yaml merge keys supported?

Glimpsing over the Yaml parser I see references to anchors/aliases at the yaml parser, which are very related features, but nothing on merge keys.
I know merge keys are not part of the 1.2 spec, only 1.1, but they are quite handy 😄

I’ll leave some details in a just for the sake of completion 🧵

Thank you!

[David Silva]

Config classes:

data class Config(val list: List<Element>)
data class Element(val a: String, val b: String, val c: String)

Yaml file (config):

defaults: &def
  a: a-value
  b: b-value
  
list:
  - <<: *def
    c: c1-value
  - <<: *def
    c: c2-value

Error:

- Could not instantiate 'Config' because:
  - 'list': Collection element decode failure (classpath:/test.yaml:5:2):
    - Could not instantiate 'Element' because:
      - 'a': Missing from config
      - 'b': Missing from config
    - Could not instantiate 'Element' because:
      - 'a': Missing from config
      - 'b': Missing from config

[sam]

The engine only supports whatever snake yml supports basically

[sam]

Did they remove merge keys in 1.2 ?

[Chris Lee]

Looks like they were removed in 1.2:

• The merge << and value = special mapping keys have been removed.

https://yaml.org/spec/1.2.2/ext/changes/#:~:text=The%20most%20significant%20difference%20between,the%20YAML%201.1%20type%20library.

[David Silva]

Yep, unfortunately.
If I recall correctly, they were meant to draft an alternative for 1.3, but I have no idea of the status tbh

[David Silva]

I’m not familiar at all with SnakeYAML, only have used it indirectly through another dependency, but I’m pretty sure I’ve used merge keys with some of those. Not sure how they manage that (maybe bind the parser to a different spec version?)

[David Silva]

Hi Sam,

Had a quick look at the parser yesterday and it is indeed supported by the snake yaml version that hoplite relies on (snakeyaml is 1.1 and snakeyaml-engine is 1.2).

However, when merge-keys are used, the projection of fields at map nodes essentially attaches the referenced map node directly to the current field being processed (<<), rather than merging its fields into the current node.
So this:

defaults: &def
  a: a-value
  b: b-value
  
list:
  - <<: *def
    c: c1-value

Sort-of turns into:

list:
  - <<:
      a: a-value
      b: b-value
    c: c1-value

Thus it won’t be able to see a and b directly on that list element, throwing the error that I’ve included above (1st message on this thread).

I’ve implemented a (quick) fix - not sure if it’s the best approach, but at least serves as a baseline to see merge-keys working.
https://github.com/sksamuel/hoplite/compare/master...davidafsilva:hoplite:feature/merge-keys-support

Would you be interested in supporting this in hoplite-yaml, or do you think it might not be worth the effort?