apple/pkl · error · VmException

cannotExportValue

cannotExportValue

Error message

cannotExportValue

What it means

Thrown when a Pkl IntSeq value (a range expression like `1...5` produced as an internal VmIntSeq) is exported to a host-language value. IntSeq is an internal representation that has no host equivalent export, so `export()` unconditionally raises cannotExportValue. Developers hit it when trying to pull such a value out of an evaluation result via the Java API.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/runtime/VmIntSeq.java:67

    return step > 0 ? start > last : start < last;
  }

  public long getLength() {
    if (isEmpty()) return 0;
    return (Math.abs((end - start) / step)) + 1;
  }

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

  @Override
  public void force(boolean allowUndefinedValues) {}

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

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

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

  @Override
  public PrimitiveIterator.OfLong iterator() {
    return new PrimitiveIterator.OfLong() {
      boolean hasNext = !isEmpty();
      long next = hasNext ? start : last;

View on GitHub (pinned to f3efcbfc9b)

Solutions

1. Convert the range to a concrete value in Pkl before exporting: `myProp = (1...5).toList()`.
2. On the host side, avoid exporting raw ranges; only export Lists, Mappings, and primitives.
3. If you control the schema, change the property type to `List<Int>`.
4. Intercept in host code and detect IntSeq-shaped results, rendering them yourself.

Example fix

// Pkl before
foo = 1...5
// Pkl after
foo = (1...5).toList()

// Pkl-side guard
assert foo is List<Int> // ensure the property is a concrete List before export

try {
  Object v = result.get("foo");
} catch (EvalException e) {
  if (e.getMessage().contains("cannotExportValue")) {
    // property is an internal value (e.g. IntSeq); fix the Pkl side
  }
}

When it happens

Trigger: Calling EvalResult.get() / export on a property whose value is an internal IntSeq (range) object instead of a rendered List.

Common situations: Binding code that reads a Pkl property defined as a range expression; host code expecting a list/array but the range was never converted (e.g. via .toList()) inside Pkl.

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

• Error converting property `%s` in Pkl object of type `%s` to
• The top-level value of a YAML stream must have type `Collect
• type constraint mismatch
• type mismatch
• type mismatch (Nothing type)

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/1ad15765d07c7371. Report an issue: GitHub.