apple/pkl · error · KotlinCodeGeneratorException

Cannot generate Kotlin code for a Pkl standard library…

Error message

Cannot generate Kotlin code for a Pkl standard library module (`${moduleSchema.moduleUri}`).

What it means

The Kotlin code generator refuses to generate code for modules whose URI scheme is `pkl` (i.e. Pkl standard library modules like pkl:base). Stdlib modules are provided by the runtime and have no user-side generated counterpart, so KotlinCodeGeneratorException is thrown from the kotlinFile property.

Solutions

1. Generate only from your own .pkl files (file:/project: URIs), not pkl: stdlib URIs.
2. Filter module list before generation: skip modules whose URI scheme is `pkl`.
3. If you need stdlib types in output, reference them from your own module rather than generating stdlib directly.

Example fix

// before
val modules = resolution.allModules // includes pkl:base
// after
modules.filter { it.uri.scheme != "pkl" }.forEach { generate(it) }

require(moduleSchema.moduleUri.scheme != "pkl") {
  "Refusing to codegen stdlib module ${moduleSchema.moduleUri}"
}

fun isGeneratable(uri: java.net.URI): Boolean = uri.scheme != "pkl"

try {
  KotlinCodeGenerator(moduleSchema, options)
} catch (e: KotlinCodeGeneratorException) {
  if (e.message?.contains("standard library module") == true) {
    logger.warn("Skipping stdlib module ${moduleSchema.moduleUri}")
  } else throw e
}

When it happens

Trigger: Passing a module URI such as pkl:base or pkl:math (or a module that imports/evaluates to a stdlib module) to KotlinCodeGenerator / `pkl codegen kotlin`.

Common situations: Scripting the codegen API and iterating over dependency modules including stdlib; accidentally targeting `pkl:` URIs instead of a project file.

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

• Standard library class
• Standard library typealias
• Cannot generate Kotlin enum class for Pkl type alias
• Cannot generate Kotlin enum class for Pkl type alias
• I/O error writing file `$outputFile`. Cause

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/6e0e16f18cc0ca1b. Report an issue: GitHub.

Appendix: source

Thrown at pkl-codegen-kotlin/src/main/kotlin/org/pkl/codegen/kotlin/KotlinCodeGenerator.kt:129

    }

  private val kotlinFileName: String
    get() = buildString {
      val (packageName, className) = nameMapper.map(moduleSchema.moduleName)
      val dirPath = packageName.split('.').joinToString("/", transform = IoUtils::encodePath)
      val fileName = IoUtils.encodePath(className)
      append("kot")
      append("lin/")
      if (dirPath.isNotEmpty()) {
        append("$dirPath/")
      }
      append("$fileName.kt")
    }

  val kotlinFile: String
    get() {
      if (moduleSchema.moduleUri.scheme == "pkl") {
        throw KotlinCodeGeneratorException(
          "Cannot generate Kotlin code for a Pkl standard library module (`${moduleSchema.moduleUri}`)."
        )
      }

      val pModuleClass = moduleSchema.moduleClass

      val hasModuleProperties = pModuleClass.properties.any { !it.value.isHidden }
      val isGenerateModuleClass =
        hasModuleProperties || pModuleClass.isOpen || pModuleClass.isAbstract

      fun generateCompanionRelatedCode(
        builder: TypeSpec.Builder,
        isModuleType: Boolean = false,
      ): TypeSpec.Builder {
        // ensure that at most one companion object is generated for this type
        val companionObjectBuilder: Lazy<TypeSpec.Builder> = lazy {
          TypeSpec.companionObjectBuilder()
        }

View on GitHub (pinned to f3efcbfc9b)