Allow defaults for declarations

[bterlson]

It would be nice to allow specifying defaults on a declaration. For example, something like:

@default(10)
scalar rating extends int8;
// or
scalar rating extends int8 = 10;

declare x = { a: string } = #{ a: "hi" };

@default(#{ x: 0, y: 0 })
model Point {
  x: float64;
  y: float64;
} = #{ x: 0, y: 0 };

Otherwise you have to make sure to apply this default everywhere rating is used, or create an anonymous model to spread in. Neither are particularly palatable.

Object defaults probably depend on the # syntax, and the sketched default decorator has similar dependencies to #514.

[markcowl]

OpenAI

[mikekistler]

Regarding defaults for objects, why not specify the defaults on the properties, e.g.

model Point {
  x: float32 = 0.0;
  y: float32 = 0.0;
}

Note: I changed x and y to float32 because of #2377

Playground link

Also, why is y quoted in the emitted OpenAPI and not x?

[bterlson]

I think the semantics are slightly different, perhaps in a too-subtle sort of way. The default on the model says, when this model is absent, here's the value to use, whereas the default on properties say, when this property is absent, here's the property value to use. The model default for example must include all required properties without defaults since it must be a valid instance of model, but this is not necessarily the case for a given model with only property defaults, because its required fields may not have defaults.

I think y is quoted because its a yaml keyword for True without quotes (short for Yes).