inputs parameters must precede emit function parameters

Error message

inputs parameters must precede emit function parameters

What it means

Sentinel error errInputPrecedence from the funcx parameter-ordering state machine: an input-side parameter kind (FnValue, FnIter, or FnReIter — side inputs and iterables) appeared after the state machine had already transitioned to the output/emit phase (FnEmit). Input parameters must all precede any emit-function parameters in a DoFn signature.

Solutions

Reorder the signature so all side inputs (func(T), func(*T) bool, func(*T,*U) bool, etc.) come before any emit function parameters

Defensive patterns

Strategy: validation

When it happens

Trigger: Thrown at sdks/go/pkg/beam/core/funcx/fn.go:539 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/22e6ae6698224738. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/funcx/fn.go:539

		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
	psWatermarkEstimator
	psType
	psInput
	psOutput
	psRTracker
	psBundleFinalization
	psStateProvider
	psTimerProvider

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