It should be possible to create Callables as const members of a class

[blackears]

Describe the project you are working on

A behavior tree library

Describe the problem or limitation you are having in your project

I'd like to create some const Callable members of a class. This seems to be impossible in GDScript at the moment.

So I'd like to be able to write

enum Status { SUCCESS, RUNNING, FAILURE }
const policy_run_until_success:Callable = func(status:Status): return status == Status.SUCCESS

At the moment this is causing the error
Assigned value for constant "policy_run_forever" isn't a constant

This does work:

enum Status { SUCCESS, RUNNING, FAILURE }
var policy_run_until_success:Callable = func(status:Status): return status == Status.SUCCESS

However, it is not a constant so it can be overwritten. The main point of this is so that I can use it as a default value in the class's constructor:

var policy:Callable

func _init(_policy:Callable = policy_run_until_success):
	policy = _policy

Describe the feature / enhancement and how it helps to overcome the problem or limitation

It allows you to create libraries of lambda functions that you can then use elsewhere.

Describe how your proposal will work, with code, pseudo-code, mock-ups, and/or diagrams

Update GDScript so that lambda functions can be assigned to const fields.

If this enhancement will not be used often, can it be worked around with a few lines of script?

No, since var fields can be overwritten and cannot be used as default parameter values.

Is there a reason why this should be core and not an add-on in the asset library?

Requires changing GDScript.

[dalexeev]

No, since var fields can be overwritten and cannot be used as default parameter values.

Note that function parameters support non-constant default values.