args(len=%d) should be the same len as numFields(%d) for: %s

Error message

args(len=%d) should be the same len as numFields(%d) for: %s

What it means

A macro evaluator defined over a struct requires exactly one route argument per struct field; when the number of supplied arguments differs from the struct's field count, the route panics with 'args(len=%d) should be the same len as numFields(%d)' (macro/macro.go:137). Variadic argument counts are not supported.

Source

Thrown at macro/macro.go:137

				return fnV
			}
		}

		return nil
	}

	numFields := typFn.NumIn()

	panicIfErr := func(i int, err error) {
		if err != nil {
			panic(fmt.Sprintf("on field index: %d: %v", i, err))
		}
	}

	return func(args []string) reflect.Value {
		if len(args) != numFields {
			// no variadics support, for now.
			panic(fmt.Sprintf("args(len=%d) should be the same len as numFields(%d) for: %s", len(args), numFields, typFn))
		}
		var argValues []reflect.Value
		for i := 0; i < numFields; i++ {
			field := typFn.In(i)
			arg := args[i]

			// try to convert the string literal as we get it from the parser.
			var (
				val any
			)

			// try to get the value based on the expected type.
			switch field.Kind() {
			case reflect.Int:
				v, err := strconv.Atoi(arg)
				panicIfErr(i, err)
				val = v
			case reflect.Int8:

View on GitHub (pinned to 7bedaf55a0)

Solutions

Supply exactly as many space-separated arguments in the path parameter as the evaluator struct has fields.
Reduce the evaluator struct to only the fields you actually pass in routes.
Update all routes that use this param type after changing the struct's field count.

Example fix

// before (evaluator struct has Min and Max)
app.Get("/{n:number 10}", h)

// after
app.Get("/{n:number 1 10}", h)

Defensive patterns

Strategy: validation

Validate before calling

// Count fields of the evaluator struct and compare with args in the path
t := reflect.TypeOf(MyEvaluator{})
nArgs := len(strings.Fields(paramArgs))
if nArgs != t.NumField() {
    log.Fatalf("param needs %d args, got %d", t.NumField(), nArgs)
}

Prevention

Provide exactly one argument per evaluator-struct field, in field order.
Grep routes after adding/removing evaluator fields and update all usages.
Document the required argument list next to the custom param type.

When it happens

Trigger: Registering a macro type whose evaluator function takes a struct with N fields, then using the type in a path with fewer or more than N arguments, e.g. {range:number 5} when the evaluator struct has two fields (min, max).

Common situations: Omitting one of the arguments (e.g. writing {ts:number 100} instead of {ts:number 100 200} for a min-max evaluator), or adding a field to the evaluator struct without updating every route using it.

Related errors

AI-assisted analysis of kataras/iris@7bedaf55a0 (2026-08-30). Data as JSON: /api/errors/5a7a6d86950e1e7b. Report an issue: GitHub.