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 0 outputs

What it means

reflectx.ToFunc3x0 converts a generic Func to a strongly-typed Func3x0 (3 inputs, 0 outputs). It panics when the underlying reflect.Type's NumIn() is not 3 or NumOut() is not 0. The Beam SDK does this to guarantee that reflection-based invocation inside the pipeline can supply exactly the expected arguments and consume no return values, failing fast with the actual type printed in the message.

Source

Thrown at sdks/go/pkg/beam/core/util/reflectx/calls.go:680

}

func (c *shimFunc3x0) Type() reflect.Type {
	return c.inner.Type()
}

func (c *shimFunc3x0) Call(args []any) []any {
	return c.inner.Call(args)
}

func (c *shimFunc3x0) Call3x0(arg0, arg1, arg2 any) {
	ret := c.inner.Call([]any{arg0, arg1, arg2})
	_ = ret
	return
}

func ToFunc3x0(c Func) Func3x0 {
	if c.Type().NumIn() != 3 || c.Type().NumOut() != 0 {
		panic(fmt.Sprintf("Incompatible func type: got func %v with %v inputs and %v outputs, want 3 inputs and 0 outputs", c.Type(), c.Type().NumIn(), c.Type().NumOut()))
	}
	if sc, ok := c.(Func3x0); ok {
		return sc
	}
	return &shimFunc3x0{inner: c}
}

func MakeFunc3x0(fn any) Func3x0 {
	return ToFunc3x0(MakeFunc(fn))
}

type Func3x1 interface {
	Func
	Call3x1(any, any, any) any
}

type shimFunc3x1 struct {
	inner Func

View on GitHub (pinned to 12126d8942)

Solutions

Make the wrapped function take exactly 3 parameters and return nothing (no error return).
If the function must return values or an error, use the appropriate converter such as ToFunc3x1/ToFunc3x2.
Verify the function's signature with reflect.TypeOf(fn).NumIn()/NumOut() before wrapping.
If extra params are unwanted, close over them in the enclosing scope so the func literal itself has exactly 3 declared parameters.

Example fix

// before
f := reflectx.ToFunc3x0(func(key string, val int, emit func(int)) error { return nil }) // returns error, panics
// after
f := reflectx.ToFunc3x0(func(key string, val int, emit func(int)) {
    emit(val)
})

Defensive patterns

Strategy: validation

Validate before calling

t := reflect.TypeOf(fn)
if t.NumIn() != 3 || t.NumOut() != 0 {
    panic(fmt.Sprintf("expected 3-in/0-out func, got %v (%d in, %d out)", t, t.NumIn(), t.NumOut()))
}
f := reflectx.ToFunc3x0(f)

Type guard

func isFunc3x0(fn any) bool {
    t := reflect.TypeOf(fn)
    return t != nil && t.Kind() == reflect.Func && t.NumIn() == 3 && t.NumOut() == 0
}

Try / catch

func safeToFunc3x0(f reflectx.Func) (out reflectx.Func3x0, err error) {
    defer func() { if r := recover(); r != nil { err = fmt.Errorf("ToFunc3x0: %v", r) } }()
    out = reflectx.ToFunc3x0(f)
    return
}

Prevention

Keep sink-style functions return-free; handle errors inside the body instead of returning them.
Add a signature assertion test for each function wrapped with ToFunc3x0.
Drop leftover (T, error) returns when converting a mapper into a consumer.
Close over extra state instead of adding parameters that change the arity.

When it happens

Trigger: Calling reflectx.ToFunc3x0(f) where f.Type().NumIn() != 3 (e.g. 2 or 4 params) or f.Type().NumOut() != 0 (the function returns 1+ values, such as an error).

Common situations: Wrapping a sink-style function that accidentally returns a value or error, registering a function with an extra context/emitter parameter added during refactoring, or pointing at the wrong converter for a 3-input function that actually returns results (should be ToFunc3x1 or higher).

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/20a523bd68cbef74. Report an issue: GitHub.