Mechanism for override file to **remove** element

[ndeloof]

What is the problem you're trying to solve
compose.yaml can be merged to override attributes, but there's no way to REMOVE an element

Describe the solution you'd like
Some way for user to explicitly define an key to REMOVE element in overridden compose model

An interesting proposal is to rely on yaml null syntax:

services:
  app:
    ports: ~

Using this, whenever the base compose file might define ports for app service, the merged model with such an explicit null attribute will have no port set.

[milas]

For ShellCommand in compose-go, we ran into something sorta similar to this - if you wanted to clear the command set via CMD (or ENTRYPOINT) in the Dockerfile, it'd get lost during marshaling.

It now distinguishes between [] and nil, which is annoying in the Go code, but works. See compose-spec/compose-go#298.

I'm mentioning because it's almost the exact opposite semantically - if the user specifies [], we'll use that value, but if they provide null (either explicitly or implicitly by not setting the field), we ignore it.

We COULD do something similar for merging such that ports: [] overrides and "clears".

[milas]

I wonder if during merging we need to have the original project in map[string]interface{} format handy and a helper like isFieldDefined("services.foo.ports") so we can only consider explicitly set fields.

[ndeloof]

also see docker/compose#8699

[ndeloof]

This indeed will require compose-go to distinguish a field being set to default value vs an explicit null assignment. We could manage this before yaml model is mapped to go structs, as a map[string]interface{} tree.
Anyway, proposal here is more to collect feedback on syntax and document this behavior