ansible array appending with set_fact

loop.yml

---
- name: looping
  shell: >
    echo "{{ loop_input }}"
  register: foo

- set_fact:
    loop_var: []
  when: loop_var is not defined

- set_fact:
    loop_var: "{{ loop_var }} + {{ [ foo.stdout ] }}"

- debug:
    var: loop_var

testing.yml

- hosts: localhost
  connection: local
  gather_facts: false
  tasks:
    - name: testing loops
      include: loop.yml loop_input={{ item }}
      with_items:
        - 'foo'
        - 'bar'
        - 'baz'
    - debug:
        var: loop_var