What causes "cannotExportValue" in apple/pkl?

Export a plain data value instead of a function (e.g. call the function and output its result).

apple/pkl · error · VmException

cannotExportValue

cannotExportValue

Error message

cannotExportValue

What it means

VmFunction.export() always throws an evalError 'cannotExportValue': function values have no externalizable representation, so forcing a function into an exported/host value (e.g. as module output or via PValue conversion) is unsupported. The input at fault is a Function-typed value placed where a data value is expected.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/runtime/VmFunction.java:179

  @Override
  public VmClass getVmClass() {
    return BaseModule.getFunctionNClass(paramCount);
  }

  @Override
  public void force(boolean allowUndefinedValues, boolean recurse) {
    // do nothing
  }

  @Override
  public void force(boolean allowUndefinedValues) {
    // do nothing
  }

  @Override
  public Object export() {
    throw new VmExceptionBuilder().evalError("cannotExportValue", getVmClass()).build();
  }

  @Override
  public void accept(VmValueVisitor visitor) {
    visitor.visitFunction(this);
  }

  @Override
  public <T> T accept(VmValueConverter<T> converter, Iterable<Object> path) {
    return converter.convertFunction(this, path);
  }

  @Override
  public boolean equals(Object obj) {
    return this == obj;
  }

  @Override

View on GitHub (pinned to f3efcbfc9b)

Solutions

Export a plain data value instead of a function (e.g. call the function and output its result).
Remove functions from the `output.value` of modules consumed externally.

Example fix

// before
output { value = toUpper }
// after
output { value = toUpper("hello") }

Defensive patterns

Strategy: validation

Validate before calling

if (typeof output.value === 'function') throw new Error('cannot export a function; call it and export the result')

Type guard

const isExportable = (v) => v === null || ['string','number','boolean'].includes(typeof v) || Array.isArray(v) || (typeof v === 'object' && !('call' in v));

Try / catch

try { module.export() } catch (e) { if (e.code === 'cannotExportValue') { console.error('output contains a function:', e.vmClass) } throw e }

Prevention

Never assign a function directly to output.value
Call functions when building the exported value
Keep exported objects free of function-typed properties

When it happens

Trigger: A module's output contains a function value, e.g. `output { value = someFunction }` or a top-level binding that resolves to a function when the module is evaluated/exported (via pkl CLI or the Java API export()).

Common situations: Assigning a function to `output.value` by mistake; exporting an object whose property is a function; accidentally referencing a function name without calling it.

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/0c488b5cd5fb2ce4. Report an issue: GitHub.