@SuppressWarnings({ "unchecked", "rawtypes" })
    private static FieldValue asArray(ArrayDataType arrType, List<FieldValue> values) {
        Array out = arrType.createFieldValue();
        for (FieldValue val : values) {
            if (val == null) {
                continue;
            }
            out.addAll((Array)val);
        }
        return out;
    }
We're either concatenating strings, or collections.
indexinglanguage/src/main/java/com/yahoo/vespa/indexinglanguage/expressions/CatExpression.java