Loops and arrays in Azure Devops Pipelines

You can create a loop in devops pipelines YAML by using a syntax similiar to -${{ each x in y }}:. From my understanding, y can be an array.

However, I find that there is no documentation for each. The only page describing an example of it's usage is on the "Templates" page.

So my question is: how do I go about specifying an array? I know one way is to use -${{ each book in parameters.books }}: and then pass in a "list" of books like:

- template: template.yml
  parameters: 
    books:
      - book1
      - book2
      - book3

However, I'd also like to define an array as a variable:

variables:
  books:
    - book1
    - book2
    - book3

However, for this, ADO throws an error A sequence was not expected.

Is there no way to define an array like that? I'd imagine I'd then refer to these as -${{ each book in variables.books }}: If not, is there any other way to specify a list?

parameters:
- name: 'param'
  type: object
  default: 
  - FOO
  - BAR
  - ZOO

steps:
- ${{ each p in parameters.param }}:
  - script: echo ${{ p }}