xai-org/x-algorithm · error · SemanticCheckFailure

Type %s does not support index accessor

Error message

Type %s does not support index accessor

What it means

The index accessor (expr[index]) supports Lists, Lists of Pairs, two-param Maps, and untyped Object expressions. Applying an index to any other static return type (e.g. Long, Boolean, StructTuple with different arity) fails semantic checking with this message.

Source

Thrown at botmaker/src/java/com/twitter/botmaker/function/collection/IndexAccessor.java:47

  public static ASTNode mkIndexAccessor(
      String exprText, ASTNode exprNode, ASTNode indexNode) throws SemanticCheckFailure {

    Type returnType = exprNode.getReturnType();
    Type indextype = indexNode.getReturnType();
    if (returnType.getBase().equals(List.class)
        && returnType.getTypeParams().size() == 1
        && indextype.equals(Type.pairOf(Type.LONG, Type.LONG))) {
      return listPairAccessor(exprText, exprNode, indexNode);
    } else if (returnType.getBase().equals(List.class)
        && returnType.getTypeParams().size() == 1) {
      return listIndexAccessor(exprText, exprNode, indexNode);
    } else if (returnType.getBase().equals(Map.class)
        && returnType.getTypeParams().size() == 2) {
      return mapKeyAccessor(exprText, exprNode, indexNode);
    } else if (returnType.getBase().equals(Object.class)) {
      return objectIndexAccessor(exprText, exprNode, indexNode);
    } else {
      throw new SemanticCheckFailure(
          String.format("Type %s does not support index accessor", returnType));
    }
  }

  private static ASTNode listPairAccessor(
      String exprText, ASTNode exprNode, ASTNode indexNode) throws SemanticCheckFailure {

    Type returnType = exprNode.getReturnType();
    Type indextype = indexNode.getReturnType();

    Signature signature = new Signature(
        ImmutableList.of(returnType, indextype),
        returnType);
    return new IndexAccessor(exprText, ImmutableList.of(exprNode, indexNode)) {
      @Override
      public Signature getSignature() {
        return signature;
      }

View on GitHub (pinned to 24c60942c5)

Solutions

1. Access struct fields with the appropriate field-access function instead of index syntax
2. Ensure maps are typed with exactly two type params (K, V)
3. Cast the expression to Object before indexing when the runtime type is genuinely indexable

Example fix

// before
someScalar[0]
// after
Get(someList, 0)

// rule-language: use explicit accessors per type
Get(list, 0) / GetByKey(map, key)

// host code before mkIndexAccessor
Type rt = exprNode.getReturnType();
boolean supports = rt.getBase().equals(List.class)
    || (rt.getBase().equals(Map.class) && rt.getTypeParams().size() == 2)
    || rt.getBase().equals(Object.class);

When it happens

Trigger: Writing myLong[0], myStruct['field'], or map[key] where the map's type params are not exactly 2.

Common situations: Using struct field syntax on a non-map, indexing a scalar returned by a previous function, or a typed map whose type parameters were lost/added.

Related errors

• Contains expects input of String, Map, Set or Collection typ
• Copy function only supports Map or StructTuple.
• StructTuple field %s expects type %s, but %s received.
• Count expects input of String, Map or Collection types but %
• expecting the input to be of String or List type but receive

AI-assisted analysis of xai-org/x-algorithm@24c60942c5 (2026-08-28). Data as JSON: /api/errors/dab1e28f0b059bcb. Report an issue: GitHub.