Creating custom tags for docker-compose files.

[mshima]

Docker compose files supports tags that does not change the yaml property type.
Example:

services:
  app:
    ports: !override
      - "8443:443" 

Would be nice to parse/stringify it using something like:

const flags = Symbol.for('yaml-flags');
const ports = ['8443:443'];
ports[flags] = ['override'].
const content = stringify({ services: { app: { ports } } });
// yaml example above

const parsed = parse(content);
console.log(parsed.services.app.ports[flags]);
// ['override']

Is this possible?

Reference:

• https://github.com/compose-spec/compose-spec/blob/5c18e329d5a15a15e4b636ed093b256b96615e33/13-merge.md#replace-value
• https://github.com/compose-spec/compose-spec/blob/5c18e329d5a15a15e4b636ed093b256b96615e33/13-merge.md#reset-value

[eemeli]

This is a bit reminiscent of the idea proposed in #294, of effectively extending Object, Array, and other values with YAML metadata. That's pretty tricky. Working with custom tags like !override is currently provided via the Document API:

import { parseDocument } from 'yaml'

const doc = parseDocument(`
services:
  app:
    ports: !override
      - "8443:443"
`)
const seq = doc.getIn(['services', 'app', 'ports'])
seq.tag // '!override'

[mshima]

Document api uses custom classes correct?
So it's not possible to our use case.

[eemeli]

Note that with your proposed use of Symbol.for('yaml-flags'), you are also defining a custom object interface.

[mshima]

With current customTag api it's not possible?

[eemeli]

With that you can specify how !override works, but you need to specify a behaviour for each tag specifically.

[mshima]

I've tried a customTag like:

{
  tag: '!override',
  identify: value => {
    console.log('identify', value);
    return value?.override === true;
  },
  resolve: value => {
    console.log('resolve', value);
    value.override = true;
    return value;
  },
},

Didn't work, identify and resolve are not called at deserialization.