xai-org/x-algorithm · error · SemanticCheckFailure

expecting the input to be of String or List type but receive

Error message

expecting the input to be of String or List type but received: %s for %s

What it means

Slice()'s semantic validation requires the first argument's static typeBase to be Object, String, or List. Other types (Map, Set, Long, etc.) fail at build time with this message, which includes the offending type and the expression text.

Source

Thrown at botmaker/src/java/com/twitter/botmaker/function/collection/Slice.java:68

    return SIGNATURE;
  }

  @Override
  protected CacheLevel getCacheLevel() {
    return CacheLevel.Global;
  }

  public Slice(String exprText, ImmutableList<ASTNode> children) throws SemanticCheckFailure {
    super(exprText, validateType(children));
  }

  private static ImmutableList<ASTNode> validateType(ImmutableList<ASTNode> children)
      throws SemanticCheckFailure {
    Class<?> inputType = children.get(0).getReturnType().typeBase;
    if (!Object.class.equals(inputType)
      && !String.class.equals(inputType)
      && !List.class.equals(inputType)) {
      throw new SemanticCheckFailure(mkErrorMessage(inputType));
    }
    return children;
  }

  @Override
  protected Object apply(
      Context<Runtime> context, Object input, Long beginIndex, Long endIndex) {
    if (input instanceof String) {
      return ((String) input).substring(beginIndex.intValue(), endIndex.intValue());
    } else if (input instanceof List) {
      return Collections.unmodifiableList(new ArrayList<>(
          ((List) input).subList(beginIndex.intValue(), endIndex.intValue())));
    } else {
      throw new IllegalArgumentException(mkErrorMessage(input.getClass()));
    }
  }

  @Override

View on GitHub (pinned to 24c60942c5)

Solutions

Convert to a List first (e.g. ToList/Set-to-list conversion) before slicing
Use ToSet+Contains or map accessors for Maps/Sets instead of Slice
Verify the first argument's return type is List or String

Example fix

// before
Slice(mySet, 0, 2)
// after
Slice(ToList(mySet), 0, 2)

Defensive patterns

Strategy: type-guard

Validate before calling

// rule-language: convert before slicing
Slice(ToList(setVal), 0, 2)

Type guard

// host code
Class<?> base = children.get(0).getReturnType().typeBase;
boolean ok = Object.class.equals(base) || String.class.equals(base) || List.class.equals(base);

Prevention

Slice only Strings and Lists
Convert Sets/Maps to Lists first

When it happens

Trigger: Calling Slice(myMap, 0, 1), Slice(someSet, 0, 1), or Slice(42, 0, 1) at rule-build time.

Common situations: Assuming Slice works on any collection; refactoring input from List to Set/Map without updating the Slice call.

Related errors

AI-assisted analysis of xai-org/x-algorithm@24c60942c5 (2026-08-28). Data as JSON: /api/errors/6ee5689deb932dc7. Report an issue: GitHub.