apache/beam · error

: failed to find required method on type

Error message

%v: failed to find required %v method on type: %v

What it means

Every CombineFn must define a MergeAccumulators method (or the passed function must be a merge-accumulators-style function). AsCombineFn looks up fn.methods[mergeAccumulatorsName]; when it's absent it fails with this error naming the fn kind (e.g. 'combineFn' or 'funType') and the type name.

Solutions

1. Add an exported MergeAccumulators method: `func (fn *MyCombine) MergeAccumulators(a, b MyAccum) MyAccum`.
2. Fix method-name spelling/casing to exactly MergeAccumulators.
3. If combining with a plain function, use a signature of the form `func(A, A) A` (merge-only).
4. Check fn.Name() in the error message to confirm you're validating the intended type.

Example fix

// before: missing method
func (c *Sum) CreateAccumulator() int { return 0 }
func (c *Sum) AddInput(a, i int) int { return a + i }
// after
func (c *Sum) MergeAccumulators(a, b int) int { return a + b }

if reflect.ValueOf(myCombine).MethodByName("MergeAccumulators").IsZero() {
    panic("MyCombine must define exported MergeAccumulators")
}

func hasMergeAccumulators(v interface{}) bool {
    return !reflect.ValueOf(v).MethodByName("MergeAccumulators").IsZero()
}

When it happens

Trigger: Passing a struct to beam.Combine that defines CreateAccumulator/AddInput/ExtractOutput but no exported MergeAccumulators method.

Common situations: Misspelling the method (MergeAccumulator, MergeAccumulatorsFn); defining it unexported (mergeAccumulators); implementing only some methods when migrating from a plain func to a CombineFn struct.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• bad combinefn
• invalid CombineFn
• MergeAccumulators must be defined on accumulator
• : requires at least 1 return value.
• bad coder kind

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/798d508fe614e8ad. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/core/graph/fn.go:1527

	if err != nil {
		return nil, errors.WithContext(errors.Wrapf(err, "invalid CombineFn"), "constructing CombineFn")
	}
	return AsCombineFn(ret)
}

// AsCombineFn converts a Fn to a CombineFn, if possible.
func AsCombineFn(fn *Fn) (*CombineFn, error) {
	const fnKind = "graph.AsCombineFn"
	if fn.methods == nil {
		fn.methods = make(map[string]*funcx.Fn)
	}
	if fn.Fn != nil {
		fn.methods[mergeAccumulatorsName] = fn.Fn
	}

	mergeFn, ok := fn.methods[mergeAccumulatorsName]
	if !ok {
		return nil, errors.Errorf("%v: failed to find required %v method on type: %v", fnKind, mergeAccumulatorsName, fn.Name())
	}

	// CombineFn methods must satisfy the following:
	// CreateAccumulator func() (A, error?)
	// AddInput func(A, I) (A, error?)
	// MergeAccumulators func(A, A) (A, error?)
	// ExtractOutput func(A) (O, error?)
	// This means that the other signatures *must* match the type used in MergeAccumulators.
	if len(mergeFn.Ret) <= 0 {
		return nil, errors.Errorf("%v: %v requires at least 1 return value. : %v", fnKind, mergeAccumulatorsName, mergeFn)
	}
	accumType := mergeFn.Ret[0].T

	for _, mthd := range []struct {
		name    string
		sigFunc func(fx *funcx.Fn, accumType reflect.Type) *funcx.Signature
	}{
		{mergeAccumulatorsName, func(fx *funcx.Fn, accumType reflect.Type) *funcx.Signature {

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