Inherited properties in TOML

explicit.toml

[thing]
name = "demo"

[thing.version_a]
items = ['base1', 'base2', 'first1', 'first2']

[thing.version_b]
items = ['base1', 'base2', 'second1', 'second2']

implicit.toml

[thing]
name = "demo"
items = ['base1', 'base2']

[thing.version_a]
items = ['first1', 'first2']

[thing.version_b]
items = ['second1', 'second2']

inherit.toml

[thing]
name = "demo"
items = ['base1', 'base2']

[thing.version_a]
items = ['+', 'first1', 'first2']

[thing.version_b]
items = ['+', 'second1', 'second2']

[thing.version_c]
items = ['third1', 'third2']

separate.toml

[thing]
name = "demo"
items = ['base1', 'base2']

[thing.version_a]
sub_items = ['first1', 'first2']

[thing.version_b]
sub_items = ['second1', 'second2']

Another suggestion that has been made to me - don't use Lists - use inline Tables:

[thing]
name = "demo"
items = {
    'base1' = True,
    'base2' = True
}

[thing.version_a]
items = {
    'first1' = True,
    'first2' = True,
}

[thing.version_b]
items = {
    'second1' = True,
    'second2' = True, 
}

[thing.version_c]
items = {
    'base1' = False,
    'third1' = True,
    'third2' = True, 
}

All inline tables would then be "union of keys".

This requires the dummy value True in the syntax, which isn't ideal. That said, In the specific use case I have for this, there might be some useful interpretations for the "value".

It also allows for partial inheritance - version_c can explicitly exclude some of it's parent values.

I like the inherit and separate solutions – it depends a bit on your target audience, though. The separate one is easy to explain and to read, and the inherit one (maybe to a slightly lesser degree), too.

I generally like the idea of explicit inherit = ["items"] configuration, but when I imagine writing a config file, I think this would actually be confusing. I wouldn't expect the inherit clause to lead to merged lists, because "inherit" sounds so all-or-nothing. "+" is more intuitive that way. Maybe this is just a naming thing though – if you don't intend to use it for primitives, "merge_with_parent" or something like that could work.

Your last proposal of inline tables strikes me as hard to use for anybody who is not an experienced developer, and hard to read when you just want to figure out the actual value of something. (Is that a use case? I think during debugging issues, it will be, regardless of the problem domain.)

+1 to inline tables being extremely hard to use, even for experienced devs.

I wouldn't expect the inherit clause to lead to merged lists, because "inherit" sounds so all-or-nothing. "+" is more intuitive that way. Maybe this is just a naming thing though – if you don't intend to use it for primitives, "merge_with_parent" or something like that could work.

What about separate extend vs inherit directives?

What about separate extend vs inherit directives?

“Extend” would do the job!