Ansible - when variable is empty or not empty

Let's say you have a playbook that should only be executed when the "foo" variable is defined and contains a value. The length filter is used to determine the length of the foo variable. 

- hosts: all
    foo: ""
    - name: fail when the 'foo' variable is undefined
       msg: the 'foo' variable is undefined
      when: foo is undefined

    - name: fail when the 'foo' variable is empty
       msg: the 'foo' variable is empty
      when: foo | length == 0


Invoking this playbook should return something like this. The ansible-doc fail command can be used to show the Ansible documention on the fail module.

When fail evaluates to true, all of the subsequent tasks are skipped.

PLAY [all]

TASK [fail when the 'foo' variable is undefined] 
skipping: []

TASK [fail when the 'foo' variable is empty]
fatal: []: FAILED => {"changed": false, "msg": "the 'foo' variable is empty"}

PLAY RECAP : ok=2  changed=0  unreacable=0  failed=1


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