apple/pkl · error · KotlinCodeGeneratorException

Pkl ` ` types are not supported by the Kotlin code…

Error message

Pkl `${this}` types are not supported by the Kotlin code generator.

What it means

toKotlinPoetName has no Kotlin mapping for certain singleton Pkl types (PType.MODULE and PType.THIS, i.e. self types). Any property typed `module` or `this` triggers this KotlinCodeGeneratorException.

Solutions

Re-type the property to a concrete class name instead of `this` or `module`.
Use an explicit parent class type: `owner: Directory` rather than `owner: this`.
Hand-write the Kotlin binding for the affected class instead of generating it.

Example fix

// before (Pkl)
class Node { parent: this }
// after
class Node { parent: Node }

Defensive patterns

Strategy: validation

Validate before calling

fun usesSelfOrModuleType(t: PType?): Boolean =
  t == PType.MODULE || t == PType.THIS ||
  (t is PType.Class && /* recurse members */ false)
// reject before codegen if found

Type guard

fun isCodegenSafeType(t: PType?): Boolean = t != PType.MODULE && t != PType.THIS

Try / catch

try {
  generator.generate(module)
} catch (e: KotlinCodeGeneratorException) {
  if (e.message?.contains("types are not supported by the Kotlin code generator") == true) {
    logger.error("Replace module/this self types: ${e.message}")
  } else throw e
}

Prevention

Avoid `this` and `module` as property types in generated modules
Use concrete class names for parent/owner references
Document self-type limitation for schema authors

When it happens

Trigger: A generated module declares a property typed `module` or `this` (self type), e.g. `owner: this` or recursive `class Foo<T extends Foo<T>>` patterns.

Common situations: Modeling parent/child relationships with self-referencing types in Pkl, then generating Kotlin bindings.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/12788504da6a5bc4. Report an issue: GitHub.

Appendix: source

Thrown at pkl-codegen-kotlin/src/main/kotlin/org/pkl/codegen/kotlin/KotlinCodeGenerator.kt:655

        }
        // Kotlin type generated for [this] is a nested enum class
        ClassName(packageName, moduleTypeName, simpleName)
      }
      else -> {
        // Kotlin type generated for [this] is a top-level type alias
        ClassName(packageName, simpleName)
      }
    }
  }

  private fun PType.toKotlinPoetName(): TypeName =
    when (this) {
      PType.UNKNOWN -> ANY_NULL
      PType.NOTHING -> NOTHING
      PType.MODULE,
      PType.THIS ->
        // TODO: support self types: `class Foo<T extends Foo<T>>`
        throw KotlinCodeGeneratorException(
          "Pkl `${this}` types are not supported by the Kotlin code generator."
        )
      is PType.StringLiteral -> STRING
      is PType.Class -> {
        // if in doubt, spell it out
        when (val classInfo = pClass.info) {
          PClassInfo.Any -> ANY_NULL
          PClassInfo.Typed,
          PClassInfo.Dynamic -> ANY
          PClassInfo.Boolean -> BOOLEAN
          PClassInfo.String -> STRING
          // seems more useful to generate `Double` than `kotlin.Number`
          PClassInfo.Number -> DOUBLE
          PClassInfo.Int -> LONG
          PClassInfo.Float -> DOUBLE
          PClassInfo.Duration -> DURATION
          PClassInfo.DataSize -> DATA_SIZE
          PClassInfo.Bytes -> BYTE_ARRAY

View on GitHub (pinned to f3efcbfc9b)