gistfile1.txt

~/src/derp> cat foo.yaml
---
- name: test
  hosts: all
#  gather_facts: false
  connection: local
  vars:
    foo:
      jar: no
      bar:
        what: red
        else: untouched

  tasks:
    - name: foo
      debug:
        var: foo

    - name: change a subelement
      set_fact:
        foo:
          bar:
            what: blue

    - name: foo
      debug:
        var: foo
        
        
  ~/src/derp> vex urs2 ansible-playbook -i hosts foo.yaml -vv
Using /Users/jkeating/src/derp/ansible.cfg as config file

PLAYBOOK: foo.yaml *************************************************************
1 plays in foo.yaml

PLAY [test] ********************************************************************

TASK [setup] *******************************************************************
ok: [haderp]

TASK [foo] *********************************************************************
task path: /Users/jkeating/src/derp/foo.yaml:14
ok: [haderp] => {
    "foo": {
        "bar": {
            "else": "untouched",
            "what": "red"
        },
        "jar": false
    }
}

TASK [change a subelement] *****************************************************
task path: /Users/jkeating/src/derp/foo.yaml:18
ok: [haderp] => {"ansible_facts": {"foo": {"bar": {"what": "blue"}}}, "changed": false}

TASK [foo] *********************************************************************
task path: /Users/jkeating/src/derp/foo.yaml:24
ok: [haderp] => {
    "foo": {
        "bar": {
            "else": "untouched",
            "what": "blue"
        },
        "jar": false
    }
}