How to check a value of an index signature schema

[diefans]

Basically I want to check the values assigned to attributes of an index signature schema:

type URI = str

schema URIs:
    foo: URI
    [key:...str]: URI

    check:
        key in ["xyz", "bar"]
        foo.startswith("http")

URIs {
    foo = "http://foo.bar"
    # FIXME: how to check this?
    xyz = "file:///tmp/test.txt"
}

...since there are no means to reference the value of the index signature, I think there is a conceptual or syntactical problem/gap. IMHO and from the fact, that there can only be one or no index signature defined, I would rather skip the customizable naming feature and would use some sort of keyword/builtin e.g. "signature" to expose key and value of the attribute e.g. signature.key and signature.value where signature itself might be Undefined in case the index signature of the schema is not defined.

@Peefy Honestly, by the way the syntax is defined, there is not much room for a named reference to the signature value - so introducing a keyword or a new syntax e.g. <name:type> or type as name seems to be an escape...

[Peefy]

Yes, this question is very good. Since keywords such as self have not yet been introduced within the schema, accessing it in the context of the schema is a bit difficult. I think it is necessary to introduce new keywords. What do you think? @zong-zhe @He1pa