may only have a single Window parameter and it must precede

Error message

may only have a single Window parameter and it must precede the EventTime and main input parameter

What it means

funcx requires a single typex.Window parameter and that it precede typex.EventTime and the main input. nextParamState returns errWindowParamPrecedence when a Window parameter is encountered after EventTime or main input, or duplicated.

Source

Thrown at sdks/go/pkg/beam/core/funcx/fn.go:531

	for i, p := range u.Param {
		if paramState, err = nextParamState(paramState, p.Kind); err != nil {
			return errors.WithContextf(err, "validating parameter %d for %s", i, u.Fn.Name())
		}
	}
	// Validate the return value ordering.
	retState := rsStart
	for i, r := range u.Ret {
		if retState, err = nextRetState(retState, r.Kind); err != nil {
			return errors.WithContextf(err, "validating return value %d for %s", i, u.Fn.Name())
		}
	}
	return nil
}

var (
	errContextParam                      = errors.New("may only have a single context.Context parameter and it must be the first parameter")
	errPaneParamPrecedence               = errors.New("may only have a single PaneInfo parameter and it must precede the WindowParam, EventTime and main input parameter")
	errWindowParamPrecedence             = errors.New("may only have a single Window parameter and it must precede the EventTime and main input parameter")
	errEventTimeParamPrecedence          = errors.New("may only have a single beam.EventTime parameter and it must precede the main input parameter")
	errWatermarkEstimatorParamPrecedence = errors.New("may only have a single sdf.WatermarkEstimator parameter and it must precede the main input parameter")
	errReflectTypePrecedence             = errors.New("may only have a single reflect.Type parameter and it must precede the main input parameter")
	errRTrackerPrecedence                = errors.New("may only have a single sdf.RTracker parameter and it must precede the main input parameter")
	errBundleFinalizationPrecedence      = errors.New("may only have a single BundleFinalization parameter and it must precede the main input parameter")
	errStateProviderPrecedence           = errors.New("may only have a single state.Provider parameter and it must precede the main input parameter")
	errTimerProviderPrecedence           = errors.New("may only have a single timer.Provider parameter and it must precede the main input parameter")
	errInputPrecedence                   = errors.New("inputs parameters must precede emit function parameters")
)

type paramState int

const (
	psStart paramState = iota
	psContext
	psPane
	psWindow
	psEventTime

View on GitHub (pinned to 12126d8942)

Solutions

Move typex.Window before typex.EventTime and the main input parameter.
Remove duplicate Window parameters.
Follow canonical order: context.Context, PaneInfo, Window, EventTime, main input.

Example fix

// before
func(et typex.EventTime, w typex.Window, v int) { ... }
// after
func(w typex.Window, et typex.EventTime, v int) { ... }

Defensive patterns

Strategy: validation

Validate before calling

for i, p := range paramTypes(fn) {
	if p == windowType && i >= mainInputIndex(paramTypes(fn)) {
		return errors.New("Window must precede EventTime and main input")
	}
}

Try / catch

if _, err := funcx.New(reflect.ValueOf(fn)); err != nil {
	return fmt.Errorf("invalid DoFn signature: %w", err)
}

Prevention

Place typex.Window before typex.EventTime and the main input parameter.
Only one Window parameter per function.
Validate signatures with funcx.New in unit tests before wiring into pipelines.

When it happens

Trigger: Signatures like func(typex.EventTime, typex.Window, int) (Window after EventTime) or func(int, typex.Window) (Window after main input).

Common situations: Adjusting DoFns for windowed processing and inserting the Window parameter at the end instead of before EventTime/main input.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/2a1d77c3efedd9dd. Report an issue: GitHub.