Is it possible to change a role variable default value according to some condition (i.e. the value of another variable)?

Details

I have two related variables for a command, env and composer_opts.

If both are left at default (env = "prod" and composer_opts = "--no-dev") everything is ok.

If I change env to dev, the default for the other one will break my command, so I always need to set both. Would it be possible to avoid this by setting a conditional default value with a custom script / if?

Important: I don't want to always set the composer_opts value according to the env value. I want to set it only if it's not already set (i.e. a dynamic default value).

Pseudocode

I would like to do something like this (following code is not valid, just pseudocode to express my need)

---
# defaults/main.yml

env: prod
composer_opts: 
    when: "{{env}}" = 'prod'
        '--no-dev --optimize-autoloader --no-interaction'
    when: "{{env}}" = 'dev'
        '' 

I suggest this solution:

---
 - set_fact:
     composer_opts: ""
   when: "{{env}}" == 'dev'

It will set composer_opts variable to string "" when variable env is equal to 'dev'.

Here is example of playbook based on updated question:

$ cat test.yml

---
- hosts: 127.0.0.1
  connection: local
  tasks:
  - set_fact:
      composer_opts: "{% if env == 'prod' %} '--no-dev --optimize-autoloader --no-interaction' {% else %} '' {% endif %}"

  - debug: var=composer_opts

Sample output:

sudo ansible-playbook test.yml -e env=dev

PLAY [127.0.0.1] ************************************************************** 

GATHERING FACTS *************************************************************** 
ok: [127.0.0.1]

TASK: [set_fact ] ************************************************************* 
ok: [127.0.0.1]

TASK: [debug var="{{composer_opts}}"] ***************************************** 
ok: [127.0.0.1] => {
    "var": {
        " '' ": " '' "
    }
}

PLAY RECAP ******************************************************************** 
127.0.0.1                  : ok=3    changed=0    unreachable=0    failed=0   


sudo ansible-playbook test.yml -e env=prod

PLAY [127.0.0.1] ************************************************************** 

GATHERING FACTS *************************************************************** 
ok: [127.0.0.1]

TASK: [set_fact ] ************************************************************* 
ok: [127.0.0.1]

TASK: [debug var="{{composer_opts}}"] ***************************************** 
ok: [127.0.0.1] => {
    "var": {
        " '--no-dev --optimize-autoloader --no-interaction' ": " '--no-dev --optimize-autoloader --no-interaction' "
    }
}

PLAY RECAP ******************************************************************** 
127.0.0.1                  : ok=3    changed=0    unreachable=0    failed=0   

While @Navern's answer does work, I found the embedded Jinja2 notation ("{% if env == 'prod' %} ...) to be extremely susceptible to notation and thus rather fragile. For example, when wrapping the line in question for better readability such as in this untested code:

composer_opts: >
               "{% if env == 'prod' %}
                   '--no-dev --optimize-autoloader --no-interaction'
                {% else %}
                   ''
                {% endif %}"

I ended up with unexpected results, such as additional whitespace or \n in composer_opts.

The approach I use is much dumber, but also more stable:

- name: set composer_opts for dev env
  set_fact:
     composer_opts: ''
     when: "{{env}}" == 'dev'

- name: set composer_opts for prod env
  set_fact:
     composer_opts: '--no-dev --optimize-autoloader --no-interaction'
     when: "{{env}}" == 'prod'

I also found this blog post to be useful which essentially follows the same approach.

Ansible set_fact based on condition in one liner :

- name: "set composer_opts based on environment"
  set_fact:
     composer_opts:  "{{ '--no-dev --optimize-autoloader --no-interaction' if (env == 'prod') else '' }}"