Ansible filter for simple text transforms of lists?

I have an Ansible variable specifing a directory, and another which is a list of files. I want to derive a list of paths comprised of each file prefixed by the directory.

vars:
  dir: /tmp
  files:
  - a
  - b
  - c
  paths: .. # some transform expression

The eventual value of paths should be as though it had been explicitly defined as

  paths:
  - /tmp/a
  - /tmp/b
  - /tmp/c

I've tried a solution with regex_replace, but that's a pretty nuclear option for such a simple transform, and I ran into endless quoting issues. I've also tried various combinations with map, join, path_join, and I don't know what-all else. Some would make perfect sense, if they didn't expect their arguments to be a mix of filter input and actual parameters.

The regex_replace example I tried seem to imply that variable definitions could 'forward reference' others in the same vars structure, but a few of my attempts gave me pages of errors suggesting that's not [always?] the case.

This is a heavily over-simplified example, and I'm looking for a more general solution than once just for the data shown.

Q: "I'm looking for a more general solution."

A: Given the data

  dir: /tmp
  files: [a, b, c]

1. The standard option is joining the products

  paths: "{{ [dir]|product(files)|map('join', '/') }}"

gives

  paths:
    - /tmp/a
    - /tmp/b
    - /tmp/c

2. The cutting-edge option is the filter community.general.path_join. The below expression gives the same result

  paths: "{{ [dir]|product(files)|map('community.general.path_join') }}"

3. But, you can easily create a general solution on your own. Write your own filters. For example,

def string_prefix(s, prefix):
    return prefix + str(s)

See string_filters.py. Create directory plugins/filter and get the filters

shell> cd plugins/filter
shell> wget https://raw.githubusercontent.com/vbotka/ansible-plugins/master/filter_plugins/string_filters/string_filters.py

Configure DEFAULT_FILTER_PLUGIN_PATH

shell> pwd
/scratch/tmp7/test-481
shell> ansible-config dump | grep DEFAULT_FILTER_PLUGIN_PATH
DEFAULT_FILTER_PLUGIN_PATH(/export/scratch/tmp7/test-481/ansible.cfg) = ['/scratch/tmp7/test-481/plugins/filter']

shell> tree .
.
├── ansible.cfg
├── hosts
├── pb.yml
└── plugins
    └── filter
        └── string_filters.py

Then, the below expression gives the same result

  paths: "{{ files|map('string_prefix', dir ~ '/') }}"

shell> cat pb.yml 
- hosts: localhost

  vars:

    dir: /tmp
    files: [a, b, c]

    paths1: "{{ [dir]|product(files)|map('join', '/') }}"
    paths2: "{{ [dir]|product(files)|map('community.general.path_join') }}"
    paths3: "{{ files|map('string_prefix', dir ~ '/') }}"

  tasks:

    - debug:
        var: paths1
    - debug:
        var: paths2
    - debug:
        var: paths3