What causes "type mismatch: value is not of type Listing" in apple/pkl?

Use Listing member syntax instead of assignment: `items { new Item { ... }; new Item { ... } }`.

apple/pkl · error · VmTypeMismatchException

type mismatch: value is not of type Listing

Error message

type mismatch: value is not of type Listing

What it means

Thrown by the Listing type-check node's executeLazily when the checked value is not a VmListing. `Listing<T>` is Pkl's heterogeneous, member-based collection type (`new Listing { ... }`); a plain List, Map, or object does not satisfy it, so the node throws typeMismatch naming the `Listing` class before delegating to the value-type check.

Solutions

Use Listing member syntax instead of assignment: `items { new Item { ... }; new Item { ... } }`.
Or build explicitly: `items = new Listing { ... }`.
If the data is genuinely a plain List, change the declared type to `List<T>`.
When converting, iterate the List and add elements into a `new Listing`.

Example fix

// before
servers: Listing<Server> = List(new Server { host = "a" })
// after
servers: Listing<Server> {
  new Server { host = "a" }
}

Defensive patterns

Strategy: type-guard

Validate before calling

assert(items is Listing<Item>, "use member syntax: items { ... }, not items = List(...)")

Type guard

items: Listing<Item> = if (items is Listing<Item>) items else new Listing {}

Prevention

Use member/amendment syntax (`prop { ... }`) for Listing-typed properties, never `=` with a List literal.
Write empty defaults as `new Listing {}`, not `List()`.
When importing JSON arrays, spread elements into a new Listing.

When it happens

Trigger: A `Listing<T>` constraint (e.g. property `items: Listing<Item>`) receives a non-Listing value: `items = List(...)` (a VmList) instead of `items { ... }` / `new Listing { ... }`.

Common situations: Assigning a `List(...)` literal to a Listing-typed property (extremely common Pkl mistake); using `= [ ... ]` bracket syntax where member syntax is required; importing JSON arrays into a Listing-typed property.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/e94c18750b72508c. Report an issue: GitHub.

Appendix: source

Thrown at pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java:1619

      return value;
    }

    @Override
    protected boolean isParametric() {
      return true;
    }
  }

  public static final class ListingTypeNode extends ListingOrMappingTypeNode {
    public ListingTypeNode(
        SourceSection sourceSection, VmLanguage language, TypeNode valueTypeNode) {
      super(sourceSection, language, null, valueTypeNode);
    }

    @Override
    protected Object executeLazily(VirtualFrame frame, Object value) {
      if (!(value instanceof VmListing vmListing)) {
        throw typeMismatch(value, BaseModule.getListingClass());
      }
      if (vmListing.isValueTypeKnownSubtypeOf(valueTypeNode)) {
        return vmListing;
      }
      return new VmListing(
          vmListing.getEnclosingFrame(),
          vmListing,
          EconomicMaps.emptyMap(),
          vmListing.getLength(),
          getValueTypeCastNode(),
          VmUtils.getReceiver(frame),
          VmUtils.getOwner(frame));
    }

    @Override
    public Object executeEagerly(VirtualFrame frame, Object value) {
      if (!(value instanceof VmListing vmListing)) {
        throw typeMismatch(value, BaseModule.getListingClass());

View on GitHub (pinned to f3efcbfc9b)