How to do a conditional ref for checkout?

I've got a Github workflow for a CI build that looks like this:

- name: Checkout meta-repo
  uses: actions/checkout@v4
  with:
    ssh-key: ${{ secrets.CI_BUILD_PRIVATE_SSH_KEY }}
    repository: meta-repo
    ref: ${{ github.event.client_payload.ref || 'dev' }}
    path: meta-repo

The github.event.client_payload.ref comes from enabling repository_dispatch - if that's not defined then we default to the dev branch. But this means when we build the main branch for this repo we use the dev branch from meta-repo which is incorrect. What I want to do is say:

1. If github.event.client_payload.ref is defined then checkout that.
2. If we're in main then checkout main.
3. Otherwise checkout dev.

I can't see a way of doing this. There's an if command but that seems to be for conditionally running entire steps. Is that the only way to achieve this?

According to GitHub Actions Operators:

GitHub offers ternary operator like behaviour that you can use in expressions. By using a ternary operator in this way, you can dynamically set the value of an environment variable based on a condition, without having to write separate if-else blocks for each possible option.

Example:

env:
  MY_ENV_VAR: ${{ github.ref == 'refs/heads/main' && 'value_for_main_branch' || 'value_for_other_branches' }}

So, you can use the ternary expression

${{ expression && true-case || false-case }}

and chain it according to your use case.

Here's an example:

- name: Checkout
  uses: actions/checkout@v4
  with:
    ref: ${{ github.event.client_payload.ref && github.event.client_payload.ref || (github.ref_name == 'main' && 'main' || 'dev') }}

A long expression may span onto multiple lines:

- name: Checkout
  uses: actions/checkout@v4
  with:
    ref: >-
      ${{ github.event.client_payload.ref && github.event.client_payload.ref
       || (github.ref_name == 'main' && 'main' || 'dev') }}

For YAML's folding and stripping with >-, See https://yaml-multiline.info/ for more details and interactive examples.