OS-dependent default-precedence variables for ansible roles

default-os-trombik.yml

# There's another method I spotted in trombik/ansible-role-dovecot. In the playbook, do this:
- name: Include OS-specific variables
  include_vars: "{{ ansible_os_family }}.yml"
  
# In {{ ansible_os_family }}.yml:
__rolename_variable: some_value

# In defaults/main.yml:
rolename_variable: "{{ __rolename_variable }}"

# This also works and might be your preferred choice if you don't like overly long lines.

default-os.yml

# Credit goes to mgedmin on freenode/#ansible for the idea.

# We use the base variable with a fixed suffix to denote the case switch.
# You should probably use something universal so that you can easily regexp
# away existing configuration.
some_variable_os:
        Debian: foo
        Redhat: bar
        default: baz

# Pull the value of some_variable_os for the current os family, with a defined fallback.
# Technically, you could also write a jinja filter that does the grunt work for you, but
# depending on what readability/maintaniblity requirements you assume, that might be extra
# work.
some_variable: "{{ some_variable_os[ansible_os_family] | default(some_variable_os['default']) }}"