templates: toJson doesn't work in some situations

[xordspar0]

Description

Imagine I'm authoring a template where I want the user of the template to pass in a map:

- name: build my thing
  template:
    name: build
    vars:
      build_args:
        time: now
        debug: true

I'd like to use the map build_args in my template like this:

steps:
  - name: build something
    image: builder
    parameters:
      build_args: {{ .build_args | mustToJson }}

But this doesn't work. It fails with this error:

json: unsupported type: map[interface {}]interface {}

In JSON, a map is only allowed to have string-type keys (map[string]any), but the Vela code that passes template parameters uses the more general map[any]any.

Value

Because toJson doesn't work, I instead use toYaml, which is apparently more tolerant of input types. It must do some runtime reflection to make sure that all the keys are strings. However, YAML is worse than JSON for this purpose. I have to indent the generated YAML properly using a hardcoded indentation value, which is prone to break when making changes to the template.

steps:
  - build something
    image: builder
    parameters:
      build_args: {{ .build_args | toYaml | nindent 14 I think? }}

JSON is better. All JSON is valid YAML, so serializing the map like this is totally fine, and much simpler.

steps:
  - build something
    image: builder
    parameters:
      build_args: {"time": "now", "debug": true}

Definition of Done

Given the example above, it should be possible to do {{ .build_args | mustToJson }} without errors.

Effort (Optional)

Impacted Personas (Optional)