Using vars_prompt with role name

[Kenneth Koski]

Apologies if this is an easy question, but I have limited experience with Ansible, and I've been unable to find anything about this online. I've got a codebase that looks like this:

# deploy.yml

---

- name: Get app name

  hosts: localhost

  vars_prompt:

    - name: app

      private: no

      prompt: "App name"

  tasks:

    - set_fact:

        app: "{{ app }}"

- name: "Setup {{ app }}"

  hosts: localhost

  roles:

    - role: "app_roles/{{ app }}"

Everything works fine if I call the playbook with 

ansible-playbook -e app=foo -i hosts deploy.yml

However, when I run it with

ansible-playbook -i hosts deploy.yml

it fails with this error, instead of prompting for the value of app

ERROR! 'app' is undefined

It looks like there's two problems here:

• The play name attribute contains "{{ app }}"
  
• The role name contains "{{ app }}"
  

The play name containing "{{ app }}" is nice but not necessary. The way it's currently architected requires the role to be dynamic, though. That leaves me with two questions:

• Is there some solution for using vars_prompt in this manner?
  
• Is there a better architecture for this?