How to remove type information with polymorphic output?

[johanvergeer]

I'm working on a dsl for Azure DevOps YAML Pipelines and they have a block called variables that can contain a few types.

variables:
  # a variable can be created in 2 different ways
  - myVar:  'myVarValue'
  - name: myVar
     value: 'myVarValue'
     readonly: true
  # a group can be created in 1 way, but always has the same key
  - group: myFirstGroup
  - group: mySecondGroup
  - group: myThirdGroup

I've been working on this for a while now and by now I have been able to create a list of Variable

@Serializable
abstract class VariableBase {
    abstract val name: String
}

@Serializable
class Variable(override val name: String, val value: String, val readonly: Boolean = false) : VariableBase()

@Serializable
data class Group(override val name: String) : VariableBase()

@Serializable
class Variables {
    var variables = mutableListOf<VariableBase>()

    fun variable(name: String, value: String, readonly: Boolean = false) {
        this.variables.add(Variable(name, value, readonly))
    }

    fun group(name: String) {
        this.variables.add(Group(name))
    }
}

val variables = Variables()
variables.variable("myVar", "myValue")
variables.group("myFirstGroup")
variables.group("mySecondGroup")
variables.group("myThirdGroup")

val result = Yaml(
    configuration = YamlConfiguration(
        polymorphismStyle = PolymorphismStyle.Property
    ),
    serializersModule = SerializersModule {
        polymorphic(VariableBase::class) {
            subclass(Variable::class)
            subclass(Group::class)
        }
    })
    .encodeToString(Variables.serializer(), variables)

print(result)

This will generate the following output:

variables:
- type: "Variable"
  name: "myVar"
  value: "myValue"
  readonly: false
- type: "Group"
  name: "myFirstGroup"
- type: "Group"
  name: "mySecondGroup"
- type: "Group"
  name: "myThirdGroup"

So how would I be able to create the list that is required to create an Azure DevOps YAML Pipeline variables block?

I guess it might be an idea to add a PolymorphismStyle.None to prevent a type output. I'll create a PR if you think that is a good idea.

[johanvergeer]

I was just able to rename the name field of the Group class to group using @SerialName("group")

@Serializable
data class Group(@SerialName("group") override val name: String) : VariableBase()

This gives the following output and brings me one step closer to the solution.

variables:
- type: "Variable"
  name: "myVar"
  value: "myValue"
  readonly: false
- type: "Group"
  group: "myFirstGroup"
- type: "Group"
  group: "mySecondGroup"
- type: "Group"
  group: "myThirdGroup"

Now I just need to get rid of the type field. Do you think my previous suggestion will work?

[johanvergeer]

The only issue I see at this moment is that this can be used to serialise. But it will be hard to deserialise when the type is not known.

Is it ok when that would be a known limitation of PolymorphismStyle.None?

[charleskorn]

Hi @johanvergeer, in this case, I'd suggest creating a custom serializer for the VariableBase.

On serialization, it could check the type of the input object and call directly into the derived class' serializer to output the value without any decoration (eg. a type field).

If you also need deserialization, it could take the Decoder instance passed to deserialize() and cast it to YamlInput and examine the YAML node directly, determine what type of variable it represents and then delegate to the appropriate derived class serializer. (This is an example of this technique - it's choosing a different path depending on the type of YAML input.)

(I'm hesitant to add support for PolymorphismStyle.None as this would only work for serialization, not deserialization.)

[charleskorn]

Hi @johanvergeer, did that work for your situation?

[johanvergeer]

Hi @charleskorn I'm pretty busy at the moment at work, so I haven't been able to get it working yet. (Also because I'm no expert so I need some figuring out 😉)

I will give it a try later this week.

[johanvergeer]

Hi @charleskorn That works like a charm. Thanks for your help. 😄