on: push
jobs:
  prepare:
    runs-on: ubuntu-latest
    steps:
    - run: |
        echo '::set-output name=matrix::{"package": ["a", "b"]}'
      id: r1
    outputs:
      matrix: ${{steps.r1.outputs.matrix}}
  evalm:
    needs:
    - prepare
    strategy:
      matrix: |-
        ${{fromJson(needs.prepare.outputs.matrix)}}
    runs-on: ubuntu-latest
    steps:
    - name: Check if the matrix key package exists
      run: |
        echo $MATRIX
        exit ${{matrix.package && '0' || '1'}}
      env:
        MATRIX: ${{toJSON(matrix)}}