xai-org/x-algorithm · error · SemanticCheckFailure

expects a Pair argument but get %s.

Error message

expects a Pair argument but get %s.

What it means

When Copy()'s first argument is map-like, every subsequent argument must be pair-like (a key/value pair produced by ToPair, typically via name: value syntax). Any non-pair child fails semantic checking with this message.

Source

Thrown at botmaker/src/java/com/twitter/botmaker/function/collection/Copy.java:58

      throw new SemanticCheckFailure("expects at least one argument.");
    }

    Type nodeType = children.get(0).getReturnType();
    if (nodeType.mapLike()) {
      return ofMap(children);
    } else if (nodeType.structTupleLike()) {
      return ofStructTuple(children);
    } else {
      throw new SemanticCheckFailure("Copy function only supports Map or StructTuple.");
    }

  }

  private static Copy ofMap(ImmutableList<ASTNode> children) throws SemanticCheckFailure {

    for (int i = 1; i < children.size(); i++) {
      if (!children.get(i).getReturnType().pairLike()) {
        throw new SemanticCheckFailure(String.format(
            "expects a Pair argument but get %s.", children.get(i).getReturnType())
        );
      }
    }

    Signature signature = new Signature(
        ImmutableList.of(Type.mapOf(TPA, TPB)),
        Type.pairOf(TPA, TPB),
        Type.mapOf(TPA, TPB)
    );

    return new Copy("Copy", children) {

      @Override
      public Signature getSignature() {
        return signature;
      }

View on GitHub (pinned to 24c60942c5)

Solutions

1. Rewrite extra arguments as name: value pairs, e.g. Copy(myMap, key: newValue)
2. Ensure each override argument is produced by ToPair (the ':' operator)

Example fix

// before
Copy(myMap, "newvalue")
// after
Copy(myMap, status: "newvalue")

// host code: verify every child after the first is pair-like
for (int i = 1; i < children.size(); i++) {
  if (!children.get(i).getReturnType().pairLike()) {
    throw new IllegalArgumentException("argument " + i + " must be a name: value pair");
  }
}

When it happens

Trigger: Calling Copy(map, someNonPairValue) — e.g. Copy(myMap, 42) or Copy(myMap, anotherMap) instead of Copy(myMap, key: value).

Common situations: Passing bare values instead of name: value pairs, or mixing map-update and struct-update syntax in one call.

Related errors

• expects <name>: <value> pairs, but a %s received
• expects at least one argument.
• Copy function only supports Map or StructTuple.
• StructTuple field %s expects type %s, but %s received.
• StructTuple expects pairs of name and values, but a %s recei

AI-assisted analysis of xai-org/x-algorithm@24c60942c5 (2026-08-28). Data as JSON: /api/errors/0f5ef12b92748ab5. Report an issue: GitHub.