Exporting constants in an emitter?

[jceipek]

I'm trying to make a TypeSpec emitter that emits constant values that come from TypeSpec definitions.

Naively, I expected consts to work for this purpose, but they seem very similar to aliases in that I can't seem to access them in an emitter.

For example, the defaultValue of field here is numeric value 10:

const MY_CONSTANT: safeint = 10;

model Test {
  field: safeint = MY_CONSTANT;
}

Trying to decorate a const produces the TypeSpec error: "Cannot decorate const statement.TypeSpec(invalid-decorator-location)".

I also tried to use enums for the same goal, but enum values can't be used in numeric contexts:

enum Constants {
  MY_CONSTANT: 10,
}

model Test {
  b: safeint = Constants.MY_CONSTANT; // "Type 'Constants.MY_CONSTANT' is not assignable to type 'safeint'TypeSpec(unassignable)"
}

Is there another way to do this?

[timotheeguerin]

Yeah you cannot decorate const as those are things resolved internally in the typespec compiler.

I think the enum case however should maybe work, could be worth fillying an issue for it.

It however works with a union like that

union Constants {
  MY_CONSTANT: 10,
}

model Test {
  b: safeint = Constants.MY_CONSTANT;
}

Playground

[jceipek]

Thanks, @timotheeguerin! I could write a decorator to convert that into an enum or bare constants.

Is there a way to use the value in a doc comment template as well?
This obviously doesn't work, but maybe something similar?

union Constants {
  MY_CONSTANT: 10,
}

model Test {
  @doc("If b is set to the default of {Constants.MY_CONSTANT}, then etc...", Constants)
  b: safeint = Constants.MY_CONSTANT;
}

I find the error Argument of type 'Constants' is not assignable to parameter of type '{}' confusing.

[jceipek]

I filed a bug for the original issue here: #4302