Default values in Listings/Mappings in combination with Subtypes

[Gr33zy]

Hello,
I try to migrate a large project using PKL. As part of the project, I have a scenario where a class hierarchy is defined:
Parent: Foo
Subtypes: Bar, Baz
So Bar and Baz have some common properties but each of those types also have a set of specific properties. In addition, there is a container class which contains, among other properties, a list of Foo. It looks something like this:

• Foo.pkl

  open module Foo
  
  name: String
  
  commonProperty: String
  
  // other properties
  // ...

• Bar.pkl

  extends "Foo.pkl"
  
  barSpecificProperty: Int
  
  // other Bar specific properties
  // ...

• Baz.pkl

  extends "Foo.pkl"
  
  bazSpecificProperty: Boolean
  
  // other Baz specific properties
  // ...

• FooContainer.pkl

  import "Foo.pkl"
  
  fooList: Listing<Foo>
  
  // other properties
  // ...

Now, I tried to create a FooContainer adding different Foo types while making use of the default property of Listings:

• foos.pkl

  amends "FooContainer.pkl"
  
  import "Bar.pkl"
  import "Baz.pkl"
  import "Foo.pkl"
  
  fooList = new Listing<Foo> {
    default {
      commonProperty = "common"
    }
    new { // Works fine
      name = "Foo1"
    }
    new Foo { // ERROR: Tried to read property `commonProperty` but its value is undefined.
      name = "Foo2"
    }
    new Bar { // ERROR: Tried to read property `commonProperty` but its value is undefined.
      name = "Bar"
      barSpecificProperty = 0
    }
    new Baz { // ERROR: Tried to read property `commonProperty` but its value is undefined.
      name = "Bar"
      bazSpecificProperty = false
    }
  }

I noticed, that as soon as I specify a concrete type like new Bar, the commonProperty will not be set by the Listing's default property. Is there any way to achieve something like this?

[HT154]

This is a limitation of Listing/Mapping defaults and is (as of recently) covered in the language reference here: https://pkl-lang.org/main/current/language-reference/index.html

One approach that works provided that you do not need to override commonProperty in specific Listing elements is to amend in that property afterwards using a member predicate:

amends "FooContainer.pkl"

import "Bar.pkl"
import "Baz.pkl"
import "Foo.pkl"

fooList = new {
  new {
    name = "Foo1"
  }
  new Foo {
    name = "Foo2"
  }
  new Bar {
    name = "Bar"
    barSpecificProperty = 0
  }
  new Baz {
    name = "Bar"
    bazSpecificProperty = false
  }
} {
  [[true]] {
    commonProperty = "common"
  }
}