How to reuse anchored entry under already unwrapped anchor? - continuous-integration

Question

I am trying to write a CircleCI config that will allow me to reuse both whole list/mapping(?) entries and its properties.
Having the following:
image_definitions:
docker:
- &default_localstack_image
image: localstack/localstack:0.10.3
environment:
KINESIS_LATENCY: 0
defaults_env: &defaults_env
environment:
PG_PORT: 5432
PG_USER: root
I would like to be able to replace:
test: &test
docker:
- image: localstack/localstack:0.10.3
<<: *defaults_env
with something like:
test: &test
docker:
- *default_localstack_image
<<: *defaults_env
but it doesn't work this way.
I've also tried:
test: &test
docker:
- *default_localstack_image
*defaults_env
but that also didn't work.
How can I do that?

Answer 1

According to the documentation:
test: &test
docker:
- <<: [*default_localstack_image, *defaults_env]
However, be aware that the merge feature is not part of the YAML spec and has only been defined for outdated YAML 1.1. I don't know if this is actually implemented. Even if it is, be aware that this merge key is the odd man out – violating the spec that says every tag is to be mapped to a type, it is instead interpreted as transformation instruction even though the loading process as defined by the spec has no place for executing transformation steps.
Similar functionality (for example for concatenating scalars) is more or less frequently requested on SO but is not available (and will probably never be) and if you need to do something like this, my advice is to do what e.g. Ansible and SaltStack do and use a templating engine as preprocessor for your YAML file.