apache/skywalking · error · IllegalArgumentException

Expected String argument for extension function, got {}

Error message

Expected String argument for extension function, got {}

What it means

Thrown by MALMethodChainCodegen.generateExtensionArg when the extension method declares a String parameter but the corresponding MAL argument is not a string literal (not a MALExpressionModel.StringArgument). Only String literals can be spliced into generated Java source for String parameters, so number literals or list literals are rejected.

Source

Thrown at oap-server/analyzer/meter-analyzer/src/main/java/org/apache/skywalking/oap/meter/analyzer/v2/compiler/MALMethodChainCodegen.java:186

        }
        sb.append(");\n");
    }

    /**
     * Generates a typed argument for an extension function call.
     * Emits raw literals: {@code 3.0}, {@code 10L}, {@code 3.0F}, {@code 10}.
     */
    private void generateExtensionArg(final StringBuilder sb,
                                       final MALExpressionModel.Argument arg,
                                       final Class<?> expectedType) {
        if (expectedType == String.class) {
            if (arg instanceof MALExpressionModel.StringArgument) {
                sb.append('"')
                  .append(DslJavaSourceText.toLiteral(
                      ((MALExpressionModel.StringArgument) arg).getValue()))
                  .append('"');
            } else {
                throw new IllegalArgumentException(
                    "Expected String argument for extension function, got "
                        + arg.getClass().getSimpleName());
            }
        } else if (expectedType == double.class || expectedType == Double.class
                || expectedType == float.class || expectedType == Float.class
                || expectedType == long.class || expectedType == Long.class
                || expectedType == int.class || expectedType == Integer.class) {
            if (!(arg instanceof MALExpressionModel.ExprArgument)) {
                throw new IllegalArgumentException(
                    "Expected number argument for extension function, got "
                        + arg.getClass().getSimpleName());
            }
            final MALExpressionModel.Expr expr =
                ((MALExpressionModel.ExprArgument) arg).getExpr();
            if (!(expr instanceof MALExpressionModel.NumberExpr)) {
                throw new IllegalArgumentException(
                    "Expected number argument for extension function");
            }

View on GitHub (pinned to 102af09b4a)

Solutions

1. Pass a double-quoted string literal for every String parameter
2. If the value is semantically numeric, change the Java parameter to double/int/long instead
3. Check for stray quotes/typos: 'myext::f(env)' parses as an expression, not a string — write myext::f("env")

Example fix

# before
expr: m.sum([]).myext::withPrefix(100)

# after
expr: m.sum([]).myext::withPrefix("100")

// pseudo: check each MAL argument kind against the Java param type before compile
boolean matches(MALExpressionModel.Argument a, Class<?> p) {
    if (p == String.class) return a instanceof MALExpressionModel.StringArgument;
    return true; // other checks for numbers/lists
}

catch IllegalArgumentException at compile time; include the extension signature in the error report

When it happens

Trigger: Extension 'f(SampleFamily, String prefix)' invoked as '.myext::f(100)' or '.myext::f(["a","b"])' — the argument node is an ExprArgument/StringListArgument instead of StringArgument. String literals must use double quotes in MAL.

Common situations: Using single quotes (MAL filter-closure style) instead of double quotes for string arguments in an extension call; passing a numeric tag value where a string key is expected; forgetting quotes entirely so the parser treats the token as an identifier expression.

Related errors

• Expected number argument for extension function, got {}
• MAL extension {}::{} expects {} argument(s), got {}
• Expected number argument for extension function
• Expected list argument for extension function, got {}
• Unknown MAL extension function: {}::{}

AI-assisted analysis of apache/skywalking@102af09b4a (2026-08-14). Data as JSON: /api/errors/9f7dcbc5877cbd0e. Report an issue: GitHub.