Simplify comprehensions for optional fields

[istvan-andrasi]

I'm playing with wrapping docker commands (build, volume/secret/config create/remove, stack deploy, ...) and I'm quite happy with what I've got so far.

For context, my simplified definition is:

#stacks: [stackname=string]: {
  volumes?: [name=string]: define?: {external?: bool, ...}
  configs?: [name=string]: define?: {external?: bool, ...}
  secrets?: [name=string]: define?: {external?: bool, ...}
}

The next thing that I'd be happy with is a simplified version of this comprehension: (x2 for configs and secrets)

if stack.volumes != _|_ {
 for name, volume in stack.volumes if volume.define != _|_ {
   if volume.define.external != _|_ {
     if volume.define.external {
       command: "\(stackname)_create_volume_\(name)"`": {...}
       command: "\(stackname)_remove_volume_\(name)"`": {...}
}}}}

I propose that inside comprehensions a trailing ? after a field name in field references can replace the explicit testing for bottom, which would result in:

for name, volume in stack.volumes? if volume.define?.external? {
  command: "\(stackname)_create_volume_\(name)"`": {...}
  command: "\(stackname)_remove_volume_\(name)"`": {...}
}

I'd like to scope this to comprehensions only as a first step.