Scalar cannot extend unknown

[mikekistler]

Cadl does not allow a scalar to extend unknown.

This should be supported.

[bterlson]

Can you say more about why?

[mikekistler]

One use case is that I can attach doc to the "unknown" (any) values. For example:

@doc("Value for parameter named by property key")
scalar ParamValue extends unknown;
model Configuration extends Record<ParamValue> {}

[bterlson]

I see. Unknown is not a scalar so I wouldn't expect this to work. I'm not sure how to declare a new type that is functionally equivalent to unknown but decoratable, though, so this may just not be supported at a deeper level. We have talked about (and resisted) decoration of aliases which would help here.

[bterlson]

I think the error you get here is not particularly helpful though, it should be more like what you get when you extend unknown with a model

[mikekistler]

Okay ... I see. I don't need to extend "unknown" in the scenario above. I can just write:

@doc("Arbitrary value for parameter named by property key")
scalar ParamValue;
model Configuration extends Record<ParamValue> {}

[bterlson]

You can def do that, the issue is that ParamValue behaves like a unit type (ala null or void) and not like unknown. E.g. if you use ParamValue in an extends clause, you it won't behave like unknown.

[markcowl]

fix error message to indicate how to create a new scalar

[markcowl]

est: 3

[mikekistler]

You can def do that, the issue is that ParamValue behaves like a unit type (ala null or void) and not like unknown. E.g. if you use ParamValue in an extends clause, you it won't behave like unknown.

@bterlson ParamValue does behave like unknown, at least for the openapi3 and autorest emitters.

openapi3 playground example: link

autorest playground example: link

[bterlson]

I was more talking about the TypeSpec side semantics around subtyping etc. But I would also argue the current emit behavior is unexpected since we would allow e.g. an object for a type declared as scalar.