apache/beam · error

value at index has type , want

Error message

value %v at index %v has type %v, want %v

What it means

createList encodes each value for the Create PCollection using the declared element type t. This error is thrown when any individual element's concrete reflect type differs from t — all values in the list must share the exact same type.

Solutions

1. Make all elements the same concrete type (e.g. wrap everything as strings, or all as int)
2. Declare the values as a typed slice (e.g. []int{...}) instead of []any
3. Convert outliers before creating: float64(x) or fmt.Sprintf as appropriate

Example fix

// before
pc, err := beam.TryCreate(s, []any{1, "2", 3.0})
// after
pc, err := beam.TryCreate(s, []int{1, 2, 3})

t := reflect.TypeOf(values[0])
for _, v := range values { if reflect.TypeOf(v) != t { return fmt.Errorf("element %v (%T) differs from %s", v, v, t) } }

func homogeneous[T any](vals []any) bool { for _, v := range vals { if _, ok := v.(T); !ok { return false } }; return true }

pc, err := beam.TryCreate(s, values)
if err != nil {
    if strings.Contains(err.Error(), "has type") { /* coerce all elements to a single type and retry */ }
    return PCollection{}, err
}

When it happens

Trigger: Calling beam.Create/TryCreate with a mixed-type []any, e.g. []any{1, "two"}, or elements whose dynamic type differs from the inferred/declared element type.

Common situations: Building test fixtures from heterogeneous literals, JSON-decoded data where numbers became float64 while other values are int, or accidentally including a nil element in the list.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• bad type: , want
• input must be a slice or array
• invalid pos type: %T
• Nested FullValues must be nested as pointers.
• passert.Diff input PColections don't have matching types

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/461f4fe8fa2134db. Report an issue: GitHub.

Appendix: source

Thrown at sdks/go/pkg/beam/create.go:93

	if len(ret) == 0 {
		t = reflect.TypeOf(list).Elem()
	} else {
		t = reflect.ValueOf(ret[0]).Type()
	}
	return createList(s, ret, t)
}

func addCreateCtx(err error, s Scope) error {
	return errors.WithContextf(err, "inserting Create in scope %s", s)
}

func createList(s Scope, values []any, t reflect.Type) (PCollection, error) {
	fn := &createFn{Type: EncodedType{T: t}}
	enc := NewElementEncoder(t)

	for i, value := range values {
		if other := reflect.ValueOf(value).Type(); other != t {
			err := errors.Errorf("value %v at index %v has type %v, want %v", value, i, other, t)
			return PCollection{}, addCreateCtx(err, s)
		}
		var buf bytes.Buffer
		if err := enc.Encode(value, &buf); err != nil {
			err = errors.Wrapf(err, "marshalling of %v failed", value)
			return PCollection{}, addCreateCtx(err, s)
		}
		fn.Values = append(fn.Values, buf.Bytes())
	}

	imp := Impulse(s)

	ret, err := TryParDo(s, fn, imp, TypeDefinition{Var: TType, T: t})
	if err != nil || len(ret) != 1 {
		panic(addCreateCtx(errors.WithContext(err, "internal error"), s))
	}
	return ret[0], nil
}

View on GitHub (pinned to 12126d8942)