Incompatible func type: got func %v with %v inputs and %v ou

Error message

Incompatible func type: got func %v with %v inputs and %v outputs, want 3 inputs and 3 outputs

What it means

reflectx.ToFunc3x3 converts a generic Func to a strongly-typed Func3x3 (3 inputs, 3 outputs). It panics when the underlying reflect.Type's NumIn() is not 3 or NumOut() is not 3. Beam throws this at conversion time so that the reflection shim's fixed return count matches the wrapped function; a mismatch would otherwise surface as an index-out-of-range or wrong-value bug deep inside the runner.

Source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:803

}

func (c *shimFunc3x3) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc3x3) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc3x3) Call3x3(arg0, arg1, arg2 any) (any, any, any) {
	ret := c.inner.Call([]any{arg0, arg1, arg2})
	_ = ret
	return ret[0], ret[1], ret[2]
}

func ToFunc3x3(c Func) Func3x3 {
	if c.Type().NumIn() != 3 || c.Type().NumOut() != 3 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 3 inputs and 3 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func3x3); ok {
		return sc
	}
	return &shimFunc3x3{inner: c}
}

func MakeFunc3x3(fn any) Func3x3 {
	return ToFunc3x3(MakeFunc(fn))
}

type Func3x4 interface {
	Func
	Call3x4(any, any, any) (any, any, any, any)
}

type shimFunc3x4 struct {
	inner Func

View on GitHub (pinned to 12126d8942)

Solutions

Make the function take exactly 3 parameters and return exactly 3 values.
Use the converter matching the real arity: ToFunc3x2 or ToFunc3x4 for 2/4 outputs.
Check with reflect.TypeOf(fn).NumIn()/NumOut() before wrapping.
If the function is part of a beam.DoFn path, prefer the typed DoFn registration API so signature matching is done by Beam's machinery instead of manual reflectx coercion.

Example fix

// before
f := reflectx.ToFunc3x3(func(a int, b, c string) (int, string, bool, error) { return a, b, true, nil }) // 4 outputs, panics
// after
f := reflectx.ToFunc3x4(func(a int, b, c string) (int, string, bool, error) { return a, b, true, nil })

Defensive patterns

Strategy: validation

Validate before calling

t := reflect.TypeOf(fn)
if t.NumIn() != 3 || t.NumOut() != 3 {
    panic(fmt.Sprintf("expected 3-in/3-out func, got %v (%d in, %d out)", t, t.NumIn(), t.NumOut()))
}
f := reflectx.ToFunc3x3(f)

Type guard

func isFunc3x3(fn any) bool {
    t := reflect.TypeOf(fn)
    return t != nil && t.Kind() == reflect.Func && t.NumIn() == 3 && t.NumOut() == 3
}

Try / catch

func safeToFunc3x3(f reflectx.Func) (out reflectx.Func3x3, err error) {
    defer func() { if r := recover(); r != nil { err = fmt.Errorf("ToFunc3x3: %v", r) } }()
    out = reflectx.ToFunc3x3(f)
    return
}

Prevention

Re-verify arity after any schema/PCollection change that adds or removes outputs.
Include (T, T, error) counting discipline in code review checklists.
Write table-driven tests that assert NumIn/NumOut for each converter target.
Wrap converters in small typed helpers so the compiler and one assertion site cover many call sites.

When it happens

Trigger: Calling reflectx.ToFunc3x3(f) where f.Type().NumIn() != 3 (e.g. 2 or 4 params) or f.Type().NumOut() != 3 (e.g. 2 or 4 return values).

Common situations: Multi-output functions that were extended to return an extra diagnostic value (4 outputs), or trimmed to drop an output (2 outputs), during schema/PCollection changes; also hitting the wrong converter after copy-pasting a ToFuncNxM call for a function with different arity.

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

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/cfb3a6bd5205f90a. Report an issue: GitHub.