How to not render empty properties?

[adam-long]

I'm trying to figure out how to prevent the rendering of a class property that is either empty or null. Basically I have an optional property I'd prefer not to be rendered if it's not set, but if it is set I want it to be one of a defined list of possible values.

class MyClass {
  name: String = "MyClass"
  value: "a"|"b"|"c"|"" = ""
}

I'd like the resulting default output to end up being:

  MyClass{
    name: "MyClass"
  }

I'd like to not need to write a custom renderer for the entire class. Any suggestions?

[StefMa]

Null is not rendered by default. So if you go with null as default, then it just works.
But I guess you can't allow any other value not to be rendered without a custom renderer...

[adam-long]

Thanks for the lead @StefMa. What I ended up doing was:

value:  "a"|"b"|"c"? = null

That at least evaluated. The odd thing was running pkl eval test.pkl ended up rendering as value = null, but running pkl eval test.pkl -f yaml omitted the property like you indicated it would. I guess output varies by type.

I didn't think that what I did was valid syntax - at least that's not what I got from the documentation on what "?" does. I thought it was syntactic sugar for performing operations on a nullable property. I guess it's also a way to define that a property is nullable.

[HT154]

The syntax for declaring nullable properties is documented here: https://pkl-lang.org/main/current/language-reference/index.html#nullable-types

Typically, when you declare an enum (well, really a literal string union) type property, you'd typically write it like this:

value: ("a"|"b"|"c")?

For nullable properties, the default value is implicitly null.

Finally, whether or not null values are rendered in the output is a property of the renderer you're using, typically via a property on it called omitNullProperties. For PcfRenderer this defaults to false (so null values are present in the output) but for JsonRenderer and YamlRenderer the default is true. If you care strongly about the output format for a module, it's often best to define the renderer in the module itself instead of relying on the -f/--format CLI arg, like so:

output {
  // always render as YAML omitting null property values
  renderer = new YamlRenderer {
    omitNullProperties = true
  }
}

You can read more about configuring module output in-language here: https://pkl-lang.org/main/current/language-reference/index.html#in-language

[adam-long]

@HT154 - that's fantastic, thanks! I missed the null entry on Type Annotations, but that explains everything quite well. And thanks for the information on omitNullProperties.