{"record":{"id":"2a1d77c3efedd9dd","repo":"apache/beam","slug":"may-only-have-a-single-window-parameter-and-it-must-precede","errorCode":null,"errorMessage":"may only have a single Window parameter and it must precede the EventTime and main input parameter","messagePattern":"may only have a single Window parameter and it must precede the EventTime and main input parameter","errorType":"validation","errorClass":null,"httpStatus":null,"severity":"error","filePath":"sdks/go/pkg/beam/core/funcx/fn.go","lineNumber":531,"sourceCode":"\tfor i, p := range u.Param {\n\t\tif paramState, err = nextParamState(paramState, p.Kind); err != nil {\n\t\t\treturn errors.WithContextf(err, \"validating parameter %d for %s\", i, u.Fn.Name())\n\t\t}\n\t}\n\t// Validate the return value ordering.\n\tretState := rsStart\n\tfor i, r := range u.Ret {\n\t\tif retState, err = nextRetState(retState, r.Kind); err != nil {\n\t\t\treturn errors.WithContextf(err, \"validating return value %d for %s\", i, u.Fn.Name())\n\t\t}\n\t}\n\treturn nil\n}\n\nvar (\n\terrContextParam                      = errors.New(\"may only have a single context.Context parameter and it must be the first parameter\")\n\terrPaneParamPrecedence               = errors.New(\"may only have a single PaneInfo parameter and it must precede the WindowParam, EventTime and main input parameter\")\n\terrWindowParamPrecedence             = errors.New(\"may only have a single Window parameter and it must precede the EventTime and main input parameter\")\n\terrEventTimeParamPrecedence          = errors.New(\"may only have a single beam.EventTime parameter and it must precede the main input parameter\")\n\terrWatermarkEstimatorParamPrecedence = errors.New(\"may only have a single sdf.WatermarkEstimator parameter and it must precede the main input parameter\")\n\terrReflectTypePrecedence             = errors.New(\"may only have a single reflect.Type parameter and it must precede the main input parameter\")\n\terrRTrackerPrecedence                = errors.New(\"may only have a single sdf.RTracker parameter and it must precede the main input parameter\")\n\terrBundleFinalizationPrecedence      = errors.New(\"may only have a single BundleFinalization parameter and it must precede the main input parameter\")\n\terrStateProviderPrecedence           = errors.New(\"may only have a single state.Provider parameter and it must precede the main input parameter\")\n\terrTimerProviderPrecedence           = errors.New(\"may only have a single timer.Provider parameter and it must precede the main input parameter\")\n\terrInputPrecedence                   = errors.New(\"inputs parameters must precede emit function parameters\")\n)\n\ntype paramState int\n\nconst (\n\tpsStart paramState = iota\n\tpsContext\n\tpsPane\n\tpsWindow\n\tpsEventTime","sourceCodeStart":513,"sourceCodeEnd":549,"githubUrl":"https://github.com/apache/beam/blob/12126d8942aaf848030c478b4c6a28c6af861c66/sdks/go/pkg/beam/core/funcx/fn.go#L513-L549","documentation":"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.","triggerScenarios":"Signatures like func(typex.EventTime, typex.Window, int) (Window after EventTime) or func(int, typex.Window) (Window after main input).","commonSituations":"Adjusting DoFns for windowed processing and inserting the Window parameter at the end instead of before EventTime/main input.","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."],"exampleFix":"// before\nfunc(et typex.EventTime, w typex.Window, v int) { ... }\n// after\nfunc(w typex.Window, et typex.EventTime, v int) { ... }","handlingStrategy":"validation","validationCode":"for i, p := range paramTypes(fn) {\n\tif p == windowType && i >= mainInputIndex(paramTypes(fn)) {\n\t\treturn errors.New(\"Window must precede EventTime and main input\")\n\t}\n}","typeGuard":null,"tryCatchPattern":"if _, err := funcx.New(reflect.ValueOf(fn)); err != nil {\n\treturn fmt.Errorf(\"invalid DoFn signature: %w\", err)\n}","preventionTips":["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."],"tags":["go","apache-beam","function-signature","reflection"],"backgroundTag":"invalid-argument-value","analyzedSha":"12126d8942aaf848030c478b4c6a28c6af861c66","analyzedAt":"2026-09-13T01:50:10.254Z","contentChangedAt":"2026-09-13T01:50:10.254Z","schemaVersion":2},"datasetVersion":"2026-09-14T16:17:12.679Z"}