apple/pkl · error · RendererException

Values of type `Duration` cannot be rendered as YAML. Value:

Error message

Values of type `Duration` cannot be rendered as YAML. Value: %s

What it means

The YAML renderer has no representation for Pkl Duration values, so visitDuration throws RendererException instead of emitting a scalar. Durations must be converted (e.g. to a number or ISO-8601 string) before rendering.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/YamlRenderer.java:144

    @Override
    public void visitInt(Long value) {
      emitter.emit(YamlUtils.plainScalar(value.toString(), Tag.INT));
    }

    @Override
    public void visitFloat(Double value) {
      emitter.emit(YamlUtils.plainScalar(value.toString(), Tag.FLOAT));
    }

    @Override
    public void visitBoolean(Boolean value) {
      emitter.emit(YamlUtils.plainScalar(value.toString(), Tag.BOOL));
    }

    @Override
    public void visitDuration(Duration value) {
      throw new RendererException(
          String.format("Values of type `Duration` cannot be rendered as YAML. Value: %s", value));
    }

    @Override
    public void visitDataSize(DataSize value) {
      throw new RendererException(
          String.format("Values of type `DataSize` cannot be rendered as YAML. Value: %s", value));
    }

    @Override
    public void visitBytes(byte[] value) {
      emitter.emit(YamlUtils.bytesScalar(value));
    }

    @Override
    public void visitPair(Pair<?, ?> value) {
      doVisitIterable(value, null);
    }

View on GitHub (pinned to f3efcbfc9b)

Solutions

1. Convert Durations in the value to plain values first (e.g. `value.toUnit(Unit.Milliseconds).value` for a number).
2. Render the duration as a string (value.toString(), e.g. "5.min") if consumers accept it.
3. Keep durations out of the rendered document, or move them to a format/renderer that supports them (e.g. PCL/XML).

Example fix

// before
renderer.renderDocument(Map("timeout", 5.min)); // throws
// after
renderer.renderDocument(Map("timeoutMs", 5.min.toUnit(DurationUnit.Milliseconds).value));

Map<String, Object> normalize(Map<String, Object> in) {
  var out = new LinkedHashMap<String, Object>();
  in.forEach((k, v) -> out.put(k, v instanceof Duration d ? d.toUnit(DurationUnit.Milliseconds).value : v));
  return out;
}

boolean isYamlRenderable(Object v) {
  return !(v instanceof Duration) && !(v instanceof DataSize) && !(v instanceof PClass);
}

try {
  renderer.renderDocument(value);
} catch (RendererException e) {
  if (e.getMessage().startsWith("Values of type `Duration` cannot be rendered as YAML")) {
    renderer.renderDocument(convertDurations(value));
  } else throw e;
}

When it happens

Trigger: renderDocument on a value tree that contains a property of type Duration (e.g. `timeout = 5.min`), reaching YamlRenderer.visitDuration.

Common situations: Rendering Pkl configs containing `5.min`, `30.s`, `1.h` values to YAML for tools that cannot understand them; output converters that pass raw Pkl values straight to the YAML renderer.

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

• Values of type `DataSize` cannot be rendered as YAML. Value:
• Maps with non-String keys cannot currently be rendered as YA
• Values of type `Class` cannot be rendered as YAML. Value: %s
• The top-level value of a YAML stream must have type `Collect
• Maps containing non-String keys cannot be rendered as JSON.

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/0b0be90cf4f17e19. Report an issue: GitHub.